It is odd that you need the PPP operation by all means, unless you are using the mobile module to dial-up to an ISP.
Normally, my experience is that, after activate the GPRS, you just need to send up one side as server (listen to a local port), and the other side as a client (connect to the remote port).
This is similar to two computers connected to same network, the only way to connect to each other with ethernet is a server-client communication.
Any comment ?