GPRS can be particularly difficult to get working in many embedded applications. As has already been mentioned, GPRS was originally intended to allow mobile devices access TO the Internet. Trying to access mobile devices FROM the Internet goes totally against the grain.

Because of NAT issues and unless you do manage to get SIM cards with static IP addresses, I think it is unlikely that you'll manage to get your two modems talking to each other directly.

An effective way of getting servers and GPRS modems all talking happily to each other is to get them all connected to a VPN server on a public static IP address. I know this works because I've done it, however I'm unable to go into any details.

In any case, establishing a VPN connection and performing tunneling on a device such as the 8052, with limited TCP/IP, is a pretty tall order. I've only used the method with Linux based systems, which are of course more than capable of performing this kind of trick.

I don't know what kind of communication you're trying to establish between the two modems, but if it is simply some sort of byte stream, then you could write a simple server application to accept socket connections from your modems and send data received on one connection through to the other. Make sure you send a small amount of data on a regular basis (maybe every 1 min?.. depends on your network provider) to keep the connections alive.

Like a VPN server, this server app. would need to bind on a fixed public IP address.

Matt.