Use FreeSWITCH in your voice stack

FreeSWITCH is an open-source softswitch.

• It answers calls and bridges them.
• It plays and records audio.
• It runs conferences and voicemail.

It works as a back-to-back user agent (B2BUA).

• A proxy such as OpenSIPS or Kamailio passes the caller’s request on to the callee.
• A B2BUA instead answers the caller’s call itself.
  • It places a second, separate call to the callee, with its own Call-ID.
  • It carries the audio of both calls through itself.

One call through FreeSWITCH is therefore two SIP dialogs, one per leg. Its audio is four streams:

• caller to FreeSWITCH,
• FreeSWITCH to caller,
• the same pair on the callee’s side.

This guide does the following:

• It builds FreeSWITCH from source.
• It sets it up to bridge a test call to a callee on the same machine.
• It proves it with a call that carries audio.

This guide does not use sipnab. When you have this working, Run sipnab beside FreeSWITCH adds sipnab.

Tested on

Every block on this page ran as written, in order, on 2026-10-12.

• The machines were clean x86_64 virtual machines with 2 cores and 3 GB of memory.
• They ran Debian 13 (kernel 6.12.63) and Ubuntu 24.04 (kernel 6.8.0).
• On each:
  • The build ran on a machine with nothing installed.
  • Run sipnab beside FreeSWITCH ran between the operate blocks in “Operate it” and the uninstall.
  • The uninstall ran last.
• Every test call carried audio both ways on both legs, counted on the wire.
• Causing each fault under When something does not work produced the message it quotes, on both.

Software	Version
FreeSWITCH	1.11.3, the current release, built from the release archive on files.freeswitch.org
sofia-sip (FreeSWITCH’s SIP stack)	1.13.18, built from the v1.13.18 tag of github.com/freeswitch/sofia-sip
spandsp (FreeSWITCH’s signal-processing library)	3.1.1, built from the v3.1.1 tag of github.com/freeswitch/spandsp
SIPp (for the test call)	the distribution’s sip-tester: 3.7.3 on Debian 13, 3.7.2 on Ubuntu 24.04

The examples use 192.0.2.10 as the machine’s address. Replace it with yours everywhere it appears.

Install FreeSWITCH

What each distribution offers

FreeSWITCH’s own packages come from SignalWire’s package repository. That repository answers only with a SignalWire account’s access token.

• Neither distribution carries a freeswitch package.
• The versions of sofia-sip and spandsp they carry are older than FreeSWITCH 1.11.3 accepts. It needs:
  • sofia-sip 1.13.18 or later,
  • spandsp 3.1.1 or later.

This block shows all three on your machine:

# Run all of these, in order.
sudo apt-get update
sudo apt-get install -y ca-certificates curl
apt-cache policy freeswitch libsofia-sip-ua-dev libspandsp-dev
. /etc/os-release
curl -s -o /dev/null -w '%{http_code}\n' "https://freeswitch.signalwire.com/repo/deb/debian-release/dists/$VERSION_CODENAME/Release"

• apt-cache policy prints nothing for freeswitch.
• It prints a Candidate of:
  • 1.12.11 for libsofia-sip-ua-dev,
  • 0.0.6 for libspandsp-dev.
• The last line prints 401. That is the HTTP status for a request that needs credentials.

	Debian 13	Ubuntu 24.04
A freeswitch package from the distribution	none	none
SignalWire’s FreeSWITCH packages	need a SignalWire access token	need a SignalWire access token
Build from source, as below	no account needed	no account needed

This guide builds from source.

• That needs no account.
• The same steps work on both distributions.
• The build installs everything under /usr/local/freeswitch.
  • sofia-sip and spandsp are the exception. They go under /usr/local.

Build sofia-sip and spandsp

First the compiler, the build tools, and the libraries FreeSWITCH’s core and its default modules use:

# Run all of these, in order.
sudo apt-get install -y --no-install-recommends git build-essential automake autoconf libtool libtool-bin pkg-config xz-utils \
  libssl-dev zlib1g-dev libjpeg-dev libtiff-dev libsqlite3-dev libcurl4-openssl-dev libpcre2-dev \
  libspeex-dev libspeexdsp-dev libldns-dev libedit-dev uuid-dev libopus-dev libsndfile1-dev liblua5.4-dev yasm nasm

Then the two libraries. They come from the FreeSWITCH project’s own repositories, at the versions FreeSWITCH 1.11.3 asks for:

# Run all of these, in order.
sudo mkdir -p /usr/local/src/voice && sudo chown "$USER": /usr/local/src/voice
cd /usr/local/src/voice
git -c advice.detachedHead=false clone --depth 1 -b v1.13.18 https://github.com/freeswitch/sofia-sip.git
git -c advice.detachedHead=false clone --depth 1 -b v3.1.1 https://github.com/freeswitch/spandsp.git
for d in sofia-sip spandsp; do (cd "$d" && ./bootstrap.sh && ./configure && make -j2 && sudo make install); done
sudo ldconfig
pkg-config --modversion sofia-sip-ua spandsp

The last line prints 1.13.18 and 3.1.1.

Build FreeSWITCH

modules.conf lists the modules the build compiles. Four of the default ones need libraries this guide does not install:

• mod_signalwire and mod_verto need SignalWire’s libks.
• mod_av needs the FFmpeg development libraries.
• mod_pgsql needs the PostgreSQL client library.

The sed line leaves those four out:

# Run all of these, in order.
cd /usr/local/src/voice
curl -fsSLO https://files.freeswitch.org/releases/freeswitch/freeswitch-1.11.3.-release.tar.xz
curl -fsSL https://files.freeswitch.org/releases/freeswitch/freeswitch-1.11.3.-release.tar.xz.sha256 | sha256sum -c -
tar xf freeswitch-1.11.3.-release.tar.xz
cd freeswitch-1.11.3.-release
sed -i -E 's#^(applications/mod_av|applications/mod_signalwire|endpoints/mod_verto|databases/mod_pgsql)$#\#\1#' modules.conf
./configure
make -j2
sudo make install
sudo ln -sf /usr/local/freeswitch/bin/freeswitch /usr/local/freeswitch/bin/fs_cli /usr/local/bin/
freeswitch -version

• sha256sum -c prints freeswitch-1.11.3.-release.tar.xz: OK.
  • The checksum comes from the same server as the archive.
  • So it detects a damaged download, not a changed one.
• The last line prints FreeSWITCH version: 1.11.3-release~64bit (-release 64bit).

The release archive’s name has a dot before -release. That is the name on the server.

Run it as a service

FreeSWITCH runs as its own user.

• This unit is the one in the FreeSWITCH source tree, debian/freeswitch-systemd.freeswitch.service, with the paths of this build.
• FreeSWITCH starts as root.
  • It raises its own scheduling priority.
  • Then it drops to the freeswitch user.
• -ncwait makes systemctl start wait until FreeSWITCH is ready to take commands:

# Run all of these, in order.
sudo useradd --system --home-dir /usr/local/freeswitch --shell /usr/sbin/nologin freeswitch
sudo chown -R freeswitch:freeswitch /usr/local/freeswitch
sudo tee /etc/systemd/system/freeswitch.service >/dev/null <<'EOF'
[Unit]
Description=FreeSWITCH
Wants=network-online.target
After=network-online.target

[Service]
Type=forking
PIDFile=/usr/local/freeswitch/run/freeswitch.pid
ExecStart=/usr/local/freeswitch/bin/freeswitch -u freeswitch -g freeswitch -ncwait -nonat
TimeoutSec=45s
Restart=always
LimitCORE=infinity
LimitNOFILE=100000
LimitNPROC=60000
LimitSTACK=250000
LimitRTPRIO=infinity
LimitRTTIME=infinity
IOSchedulingClass=realtime
IOSchedulingPriority=2
CPUSchedulingPolicy=rr
CPUSchedulingPriority=89
UMask=0007

[Install]
WantedBy=multi-user.target
EOF
sudo systemctl daemon-reload
sudo systemctl enable freeswitch

-nonat stops FreeSWITCH from asking the local router to forward ports to it (UPnP and NAT-PMP).

Configure FreeSWITCH

make install put FreeSWITCH’s example configuration in /usr/local/freeswitch/conf. The FreeSWITCH project calls it “vanilla”. It runs two SIP profiles. Each profile is a SIP listener with its own settings:

• internal, on port 5060, for your own phones.
  • It asks every caller for a user name and password.
• external, on port 5080, for carriers and other servers.
  • It asks for no password.
  • It sends each call to the dialplan’s public context.

The test call in “Place a test call” goes to external. This block changes five things:

# Run all of these, in order.
cd /usr/local/freeswitch/conf
sudo sed -i 's|=stun:stun.freeswitch.org"|=192.0.2.10"|' vars.xml
P=$(od -An -tx1 -N12 /dev/urandom | tr -d ' \n')
sudo sed -i "s|data=\"default_password=1234\"|data=\"default_password=$P\"|" vars.xml
sudo sed -i 's|<param name="listen-ip" value="::"/>|<param name="listen-ip" value="127.0.0.1"/>|' autoload_configs/event_socket.conf.xml
sudo sed -i -E 's#^( *)(<load module="mod_(verto|signalwire|av)"/>)#\1<!-- \2 -->#' autoload_configs/modules.conf.xml
sudo tee dialplan/public/01_echo.xml >/dev/null <<'EOF'
<include>
  <!-- A call to "echo" goes to the test callee on this machine, in PCMU. -->
  <extension name="test_callee">
    <condition field="destination_number" expression="^echo$">
      <action application="bridge" data="{absolute_codec_string=PCMU}sofia/external/echo@127.0.0.1:5070"/>
    </condition>
  </extension>
</include>
EOF
sudo chown -R freeswitch:freeswitch /usr/local/freeswitch/conf
grep 'stun-set' vars.xml
sudo systemctl restart freeswitch
fs_cli -x 'sofia status'

What the changes do:

• The machine’s address.
  • external_rtp_ip and external_sip_ip are the addresses FreeSWITCH puts in its SIP and SDP for the external profile.
  • The example configuration asks the public STUN server stun.freeswitch.org for them at startup.
  • That gives the address of your internet connection, not of this machine.
  • With that default, the Contact header of FreeSWITCH’s replies to the test call named the public address.
  • On a machine with its own public address, put that address here.
• The SIP password.
  • default_password is the password of the 20 example users, 1000 to 1019, that the internal profile accepts.
  • The example configuration sets it to 1234.
  • The block replaces it with a random one.
• The event socket.
  • fs_cli is FreeSWITCH’s command-line client.
  • It talks to FreeSWITCH over its event socket, TCP port 8021, with the password ClueCon.
  • The example configuration listens on every address.
  • The block moves it to 127.0.0.1, so only this machine reaches it.
• Modules the build left out.
  • The example configuration loads mod_verto, mod_signalwire and mod_av.
  • The section “Install FreeSWITCH” did not build them.
  • Without this line, FreeSWITCH logs Error Loading module for each at every start.
• Where the test call goes.
  • The dialplan is FreeSWITCH’s routing.
  • FreeSWITCH bridges a call to echo in the public context to a test callee on this machine, 127.0.0.1:5070.
  • absolute_codec_string=PCMU makes FreeSWITCH offer the callee PCMU only. That is the one codec SIPp’s callee answers with.

sofia status lists the profiles. Both are RUNNING:

• internal shows sip:mod_sofia@192.0.2.10:5060.
• external shows sip:mod_sofia@192.0.2.10:5080.

Place a test call

SIPp plays both ends:

• a callee on this machine that echoes audio back,
• a caller that calls echo on FreeSWITCH’s external profile and plays a recorded G.711 sample.

So the call carries real audio both ways. The caller uses port 5090, because FreeSWITCH holds 5060 and 5080:

# Run all of these, in order.
sudo apt-get install -y sip-tester jq
mkdir -p ~/sipp/pcap && cd ~/sipp
ln -sf /usr/share/sip-tester/*.pcap pcap/
sipp -sn uas -i 127.0.0.1 -p 5070 -rtp_echo -m 1 -bg
sudo sipp -sn uac_pcap 192.0.2.10:5080 -i 192.0.2.10 -p 5090 -s echo -m 1 -timeout 90s

• The caller needs sudo because it plays the audio sample through a raw socket.
• It sends about 7 seconds of audio, then a short DTMF clip, and hangs up.
• At the end SIPp’s statistics screen shows Successful call at 1.

SIPp’s built-in caller sends its ACK and BYE straight to FreeSWITCH. FreeSWITCH is the far end of the caller’s dialog. Unlike the proxy guides, this guide needs no edited scenario.

Each leg has its own audio.

• The caller offers PCMA, and FreeSWITCH answers it in PCMA.
• On the callee’s leg FreeSWITCH offers PCMU.
• FreeSWITCH converts the audio between the two codecs.

Run sipnab beside FreeSWITCH shows all four streams.

Operate it

See a call’s two legs while it is up. FreeSWITCH calls each leg a channel. Start a call in the background and list the channels:

# Run all of these, in order.
cd ~/sipp
sipp -sn uas -i 127.0.0.1 -p 5070 -rtp_echo -m 1 -bg
sudo sipp -sn uac_pcap 192.0.2.10:5080 -i 192.0.2.10 -p 5090 -s echo -m 1 -timeout 90s -bg
sleep 3
fs_cli -x 'show channels as json' | jq -r '.rows[] | "\(.direction) \(.name) \(.read_codec)"'
sleep 12

inbound sofia/external/sipp@192.0.2.10:5090 PCMA
outbound sofia/external/echo@127.0.0.1:5070 PCMU

• The inbound channel is the caller’s leg, in PCMA.
• The outbound one is the callee’s, in PCMU.
• The last line waits for the call to end.

Check health and read the logs.

# Run all of these, in order.
systemctl is-active freeswitch
fs_cli -x status
sudo tail -n 30 /usr/local/freeswitch/log/freeswitch.log

• fs_cli -x status prints:
  • FreeSWITCH’s version,
  • its uptime,
  • the number of sessions since it started.
• FreeSWITCH writes its log to /usr/local/freeswitch/log/freeswitch.log, not to the journal.

Apply a configuration change.

• A change to the dialplan or the users takes effect with reloadxml.
• A change to vars.xml or a SIP profile needs a restart:

# Run all of these, in order.
fs_cli -x reloadxml
sudo systemctl restart freeswitch
fs_cli -x status | head -2

reloadxml prints +OK [Success].

Uninstall. Remove FreeSWITCH, its user, and the two libraries:

# Run all of these, in order.
sudo systemctl disable --now freeswitch
sudo rm -rf /usr/local/freeswitch /etc/systemd/system/freeswitch.service /usr/local/bin/freeswitch /usr/local/bin/fs_cli
sudo systemctl daemon-reload
sudo userdel freeswitch
sudo make -C /usr/local/src/voice/sofia-sip uninstall
sudo make -C /usr/local/src/voice/spandsp uninstall
sudo rm -rf /usr/local/include/sofia-sip-1.13 /usr/local/include/spandsp /usr/local/share/sofia-sip
sudo ldconfig
rm -rf /usr/local/src/voice/sofia-sip /usr/local/src/voice/spandsp /usr/local/src/voice/freeswitch-1.11.3.-release*

When something does not work

• The test call fails, and SIPp reports Aborting call on unexpected message ... received 'SIP/2.0 488 Not Acceptable Here.
  • The callee answered with a codec FreeSWITCH had not offered it.
  • Without absolute_codec_string, FreeSWITCH offers the callee the caller’s codec, PCMA.
  • SIPp’s callee answers PCMU whatever FreeSWITCH offers.
  • FreeSWITCH’s log shows Originate Failed. Cause: INCOMPATIBLE_DESTINATION.
  • Keep {absolute_codec_string=PCMU} in the bridge line, or offer your callee a codec it supports.
• fs_cli prints [ERROR] fs_cli.c:1699 main() Error Connecting [].
  • Nothing answers on the event socket. One of these is true:
    • FreeSWITCH is not running.
    • Its listen-ip is not the address fs_cli connects to, 127.0.0.1.
  • Check systemctl status freeswitch.