FreeSWITCH expert VOIP

I have an occasionally issue with audio between Freeswitch and our PBX with PBX (Sip Wise) . CAll enter Freeswitch and then continue to PBX. This issue is that we cannot hear a greetings from our PBX and it looks more random since in most of the cases it is working fine.
We are looking for expert who can analyze and fix the issue within the Freeswitch.