I did "some" playing when my phone has a real modem in it. It was basically a PPP internet connection. I made/modified a modem driver for Mac OS9 for a Cell phone.
In the same time frame, I could have had a "data" number and a "cell" number. In order to call a FAX machine, you had to have "CSD" enabled, otherwise the end drops the call. That is limited to 9600 baud.
The problem will probably be routing to the other phone. I did some other PC related unsuccessful playing where I was trying to aggregate a restricted link (http only) with my cell link and have the non-permitted traffic use the cell. That didn't work. I was using the APN of the phone and, I think, that was part of the problem. The connection was limited to one.
Connections drop if they are not sustained.
We do have a service in the US called clear.
www.Clear.com
I never tried calling the phone. Maybe I did. Not sure. Too long ago.
I know I could figure out the IP I was assigned. e.g.
www.whatismyIP.com will work. But you may need a Dynamic DNS service to find the other modem.
Not much help?