wachkar wrotefacing the same problem, although my iphone was activated by default using its original AT&T sim.
the issue seems to be related to Alfa and MTC as they do not give priority to voice over data on iphones.
I second that. I got my iPhone 4 locked from the US and activated it with an AT&T SIM card, I'm now using it with Alfa and facing the issue of not receiving calls when using EDGE. It's normal behavior for the data to be a priority (at least on Alfa), however, the main issue is that even if I disable push, I still get things like:
18:53:17.477218 IP (tos 0x0, ttl 64, id 46825, offset 0, flags [DF], proto TCP (6), length 121) 172.23.129.139.61394 > st11p01st-courier036-01.[b]push.apple.com[/b].5223: P 1645:1714(69) ack 3695 win 32832 <nop,nop,timestamp 618050595 364133495>
18:53:17.998326 IP (tos 0x0, ttl 46, id 60042, offset 0, flags [DF], proto TCP (6), length 52) st11p01st-courier036-01.push.apple.com.5223 > 172.23.129.139.61394: ., cksum 0x2b89 (correct), 3695:3695(0) ack 1714 win 90 <nop,nop,timestamp 364133954 618050595>
18:53:18.016720 IP (tos 0x0, ttl 46, id 60043, offset 0, flags [DF], proto TCP (6), length 89) st11p01st-courier036-01.push.apple.com.5223 > 172.23.129.139.61394: P 3695:3732(37) ack 1714 win 90 <nop,nop,timestamp 364133954 618050595>
Which occupy the connection and lead to people not being able to reach me.
I got the above dump by running tcpdump in verbose mode on the
pdp_ip0 interface.