Zap channel randomly not detecting asnwer

I’m experiencing strange behavior while placing outgoing calls from SIP phone to PSTN via Zap channels (TE410P over E1). I have simple dialplan to make outcalls:

[quote]exten => _XXXXXXX,1,Set(CALLERID(number)=445696266)
exten => _XXXXXXX,n,Dial(Zap/g1/${EXTEN},r)[/quote]


Incoming calls do work fine, and outgoing calls randomly fail like that: I pick up the hook on the PSTN phone and there is silence there while * says it’s still ringing and SIP phone also continues to ring. But sometimes the call go through fine. I don’t do any reloads etc, just place one call and then another.
pri debug span 1 for successful and unsuccesfull calls showed that sometimes * doesn’t receive CONNECT (7) and CONNECT ACKNOWLEDGE (15) messages, i.e. for failed call the following part is absent:

I didn’t see any regularity in this behavior. Please help, cause I’m pretty stuck with it :frowning: