this application allows calling or responding to a SIP "data" call (see RFC4040) Without going deep into the rabbithole of ISDN, this application implements only synchronous ppp in hdlc-like framing (RFC1662)

It is not intended to develop this to a fully fledged all corner case handling application but exists only because of having a Fritz!Card and Fritz!Box with S0 and having seen https://www.youtube.com/watch?v=rQfy8T-VOs4 from "The Serial Port" channel

V.110, V.120 or X.75 is not implemented.

V.110 might be useful for this to be a GSM dialup server (or calling a ppp server that runs over GSM)

I don't see a need for X.75 since as far as I know usually ppp/internet was set up mostly using RFC1662

The reason why this application does not support emulating a AT modem is because this application will only work with ppp and not dealing with AT commands too calling pppd directly was choosen.

To use ipv6 instead of ipv4 for sip and rtp transport, just add --ipv6 to the command line If you don't want to register to a pbx, just don't set --reg, --user and --pass. If --bindport is not set the application will use a random port (not 5060) to bind itself to. If you want some consistency just set this value to something you find appropriate You can use --srtp to enable opportunistic srtp, if you call (--dial) or register (--reg) with sips: or sip:...;transport=tls, srtp will be mandatory

In doubt see the source code, but in general something like this on the "server" side (as root because of pppd)

ppp-sip-isdn --loglevel 1 --id sip:number@pbx.server --reg sip:pbx.server --user number --pass XXXXXXXXXXX --pppd "nodetach debug refuse-eap refuse-chap refuse-mschap require-mschap-v2 local proxyarp ms-dns 1.1.1.1 mp" --bindport 44444 --ppplocalip 10.0.1.1 --pppremoteipstart 10.0.1.2 --linecount 8

and on the "client" (channel bind 2 calls, as root because of pppd)

ppp-sip-isdn --loglevel 1 --id sip:number@pbx.server --reg sip:pbx.server --user number --pass XXXXXXXXXXX --pppd "nodetach debug noauth user myuser password mypassword mp" --bindport 44443 --linecount 2 --dial sip:dialinnumber@pbx.server

one one side (server)

ppp-sip-isdn --loglevel 1 --id sip:serverside@localhost --pppd "nodetach noauth mp" --bindport 5060 --ppplocalip 10.0.1.1 --pppremoteipstart 10.0.1.2 --linecount 2

on the other side (client)

ppp-sip-isdn --loglevel 1 --id sip:clientside@localhost --pppd "nodetach noauth mp" --bindport 5060 --dial sip:serverside@ip-of-server --linecount 2

you should get 2*64kbps. Once maybe that felt blazing fast, but except for nostalgia, this speed is useless today.

cd ppp-sip-isdn
git submodule init
git submodule update
make