Asterisk + FreePBX - One-way audio
I use CentOS 6.7 droplet and Asterisk 12.8.2 + FreePBX 184.108.40.206 (I install it according to instructions http://wiki.freepbx.org/display/FOP/Installing+FreePBX+12+on+CentOS+6.5). Calls are made with the use of sip-trunk from https://zadarma.com/en/.
When the caller dials the extension number and forwarded to an external mobile number, sometimes occurs a one-way audio. The called party cannot hear the caller.
Do you have some experience in dealing with such problems? What do you recommend in this situation?