Incoming VoIP to VoIP drops the call

[maxxer]

Hi.
We're experiencing a strange issue on our on-premise installation: it works great, but when we receive a phone from another VoIP system the call drops after two rings, but on the other side they say there's no answer.

Our 3CX has an IVR, so the calling user listens to the message, composes the number to dial phones, and to them it rings forever, while to our phone just makes two rings. On the 3CX call log I see the call as unanswered.
If the call comes from a traditional PSTN line this won't happen.

What could it be the problem? A missing portforward?
thanks

 

[leejor]

when we receive a phone from another VoIP system the call drops after two rings, but on the other side they say there's no answer.

Sounds like a SIP message is not getting through, one way or another.

"another VoIP system" means what? Your VoIP provider, a trunk from another PBX? Since you mention PSTN lines, is that how you connect to outside trunks?
Does the 3CX Firewall check pass?
Does the 3CX activity logs provide any detail about the dropped calls when compared to a call that works, what is the difference?
Have you tried using Wireshark to see what message, or lack of, is happening at the "two ring" point?
Did this issue just start, or has it been this way from the beginning? If it just started, what has been changed?

 

[maxxer]

"another VoIP system" means what? Your VoIP provider, a trunk from another PBX? Since you mention PSTN lines, is that how you connect to outside trunks?

Another VoIP system means another trunk, from a different provider than mine.
I mentioned PSTN lines because if I make a call from a traditional phone system (i.e. my cell phone) everything works fine.

Does the 3CX Firewall check pass?

yes

Does the 3CX activity logs provide any detail about the dropped calls when compared to a call that works, what is the difference?

as I said 3CX reports the call as unanswered for all the time the caller waits for an answer

Have you tried using Wireshark to see what message, or lack of, is happening at the "two ring" point?

no, not yet. I first asked there to know if there was a known/obvious cause for this behavior. If necessary, I'll debug the call

Did this issue just start, or has it been this way from the beginning? If it just started, what has been changed?

It probably has been there since we deployed 3CX. We don't have many callers from VoIP lines, so when the first reported me the issue I thought it was a problem on their side, since for everyone else we had no problems. Now I had a second report, who tried reaching me twice and in both times it behaved the same, so it I realized it's a problem on our side.

 

[leejor]

This is all happening from one VoIP provider, or do you have a second to test over?

The fact that callers continue to hear ring-back tone when 3CX has dropped the call, really suggests that a SIP message is not getting through.

 

[maxxer]

It's happening with at least two different VoIP providers.

It seems that 3CX hasn't dropped the call, as in the logs I see not answered...

I'll do some intensive tests tomorrow to see if I can get any error

 

[leejor]

There must be a message generated as to why it stops ringing (times out?) after two rings, yet the logs say that it is "unanswered". Have you enabled more detailed information in the 3CX logs?
If the call is answered during the two rings, does the call stay up?