GPRS Vs EDGE - Trying to decode my own sessions

Hi all, i'm testing gprsdecode with osmocomBB and using the samples provided from Nohl works well. Using a Blackberry 8300 and my Wind's sim, i tried to dump some my internet sessions.  Unfortunately even if i'm locking the MS to a fixed ARFCN, it is "upgrading" from GRPS to EDGE very quickly and i cannot dump an entire session and then decode it on gprsdecode. The only datas i could decode and view on wireshark are few "malformed packets". As i suppose the problem is that i'm not able to fix the MS to work only on gprs. Which phone did Luca use to make his tests? (just a simple one that was not EDGE capable? ) Which was  the successful-rate of (third-party) decoded sessions? I'm asking it,  because i'm not even able to "see" other's phones sessions...  ok, could be possible that everyone else is using only EDGE... but seems strange. Have you some hints for that? Thank you for attention. Regards, Luca