I'm still not able to sniff enough data to reconstruct TCP sessions.
I can get datagrams (even TCP), but they look like "sparse" datagrams. Even using 2 sniffing phones I have a slightly better result, but not enough to consider it satisfying.
Are there some other steps that can be done? 

Is there anyone, other that gprs decoder authors, able to make it completely working?

Thanks.
Dario.