I have 7920 phones (firmware 3.01) in remote sites. In central site i have CCM 5.04 with gateway cisco 2821 (IOS 12.4.11T) to connect to PSTN (one E1 connection and 2 BRI connections connected to GSM interfaces)
Beetween sites (regions)ip phones talk G.729.
From 7920 phones when i call to mobiles phones trough BRI connection in 2821 at central site i hear a strange noise during call like a clock! If i call from a 7912 phone i don't have problems.
If i call from 7920 to PSTN through E1 connection in 2821 i don't have problems.
And if i change region of 7920 ip phone so he can talk G711 i don't have problems.
During call i see in GW that G729r8 is used.