My ISP offers to his VDSL users the ability to use any smartphone as a cordless home phone, using an official app.
They provide no documentation or source code, but someone managed to de-obfuscate the app and found out that it works thanks to a SIP server that runs on the modem/router the ISP lends to their customers. There is no way to configure it, it's completely hidden to the average user.
This guy also found the way the app obtains the credentials needed for the login, thus enabling any device with a SIP client installed on it to connect to the router and receive and make calls on the land line.
The address the app uses for the login is a string (modemtelecom.homenet.telecomitalia.it
) that is resolved to the router's LAN address (by default 192.168.1.1
). If I try to login to [email protected]
it works fine; anyway, if I attempt to log in to the server, using as host address my external IP address, it fails.
I tried to set the router to forward traffic incoming from the WAN on the SIP port to its own address, but it refuses to do so (I don't know if this may be a restriction from the ISP or something one just can't do).
Do you think there may be a way to trick my router into letting me login to the SIP server while I'm outside my home LAN?
As MariusMatutiae suggested me, I set up a OpenVPN server on a PC connected to my LAN; I then proceeded to masquerade the traffic from OpenVPN, and successfully connected my Android smartphone to the VPN and the SIP server inside my LAN. So far though it seems like I can only send audio, without being able to receive anything.
Anyways, this looks like the right track: to try to get help with the issues I encountered, I asked another question here.