[Asterisk-Users] iax bracking up

[Altus Syman]

Good day all
We have a 64kbit 128kbit burst able line between 2 city's
iax.conf is configured so that the SIP users in one office can just dial 
the extension of the other office.
the codec is ilbc and I added trunk=yes.
When you call someone on the other side its clear to you but the other 
side brakes up?
Does someone know any better tricks or tips for this scene
Thanks
Altus