IAX trunk codec problem: only ALAW?

Hi all,

I have 2 asterisk versione 1.4.5 connected via VPN and a IAX trunk for calls
between the 2 PBX.
The trunk works well if I use the ALAW codec, but if I try to use any other codec like GSM or ILBC o G729 I obtain the following error in /var/log/asterisk/messages of the PBX receiving che call:

[Sep 8 19:44:44] WARNING[3803] channel.c: Don’t know any of 0xe000 formats
[Sep 8 19:44:44] ERROR[3803] chan_iax2.c: No best format in 0xe000???

I need to change the codec because I need to reduce the bandwidth usage due to low speed ADSL :frowning:

Any help will be very appreciated!