Intermittent connection with asterisk

I’ve been using asterisk from about 2 years now. I’ve setup both asterisk system here at our office. Every thing works well, configuration to partners are all oki. My problem is like this. I’ve tried making some interconnection with other itsp at Bahrain. All setup,configuration were oki.Both system can make some calls.It means that I have configured the system correctly. The thing is sometimes incoming calls from system coming from bahrain was not successful. Calling party on Bahrain just does not heard any ringbacktones it’s just DEAD air or total silence. But there were times that ALl calls were being accpeted by our system. (I can see on the CDRs).

We have replicated the problems by making some test calls. It happened once again. I’m just wndering when the failed calls occur, there were no logs on the CLI mode, and some sip messages are missing during the test calls. But on successful calls, we have complete sip message including the invite,trying,ringing,ack.,etc.

I was thinking if these are internet related isssues. Our connection to our partners in Bahrain is thru internet or IP.
I hope you can help me our problems cause I’m a little stuck in here.
Thank you very much

Best regards,

Are you behind NAT ?
When the calls fail do you see anything in the CLI ? I would run NGREP and see if any packets are coming in when the calls are failing. If nothing comes in then it is the provider in Baharain’s issue. If the calls go through just no ring tones I would try ringing on your end with MOH and see if the calling party hears anything. If they don’t then most likely the I(T)SP in Bahrain is blocking early media.