Freepbx gsm gateway call via trunk-outbound route and one way audio?

Scenario is, 8000 sip extension try to make gsm call via gsm gateway. But there is one way audio.
8000 can’t hear any thing. But gsm phone hears 8000.
Created custom trunk for calling via GSM gateway than outbound route is applied to this trunk.
What can be wrong ?
Two sip clients can speak eachother without any problem. But there is a way audio in GSM call.