Incoming call does not get through: second try rings the extension

[korken]

VOIP provider is Deutsche Telekom, SIP trunk is green, running 3CX 15.5SP5 - calling one of their DIDs from outside is sometimes not being processed at all - no phone is ringing and no log entries are written whatsoever.
Solution is atm:

1) Calling a 2nd time again comes through (not sure)
2) Calling out my smartphone 'opens' the line again - calling in rings the proper extension immediately

I am sure the error description is not that sufficient, so please feel free to ask for further details. How must I set up the 3CX system so that the inside extensions are reachable from the outside at all time?

 

[YiannisH_3CX]

Hello @korken

On the first call that is not working do you see the call arriving to the PBX? Is there any entry of the call in the Activity log?

 

[korken]

Hello @korken

On the first call that is not working do you see the call arriving to the PBX? Is there any entry of the call in the Activity log?

Thank you for your reply!

No, I am not seeing any according entries in the activity log.

 

[leejor]

If you are not seeing an activity log entry on the incoming calls, make note of the times it happens and and engage your provider. They may be able to see what happened to the calls.

Is it always calls to the same number, or calls to various numbers over the same trunk?

 

[korken]

Is it always calls to the same number, or calls to various numbers over the same trunk?

It's happening to all DIDs belonging to that trunk. Well, I will write down some of those ocassions and discuss this with my provider.
I wanted to exclude any apparent errors on my side like stun servers, fix/dynamic public ip, keep alive, TTL and so on :-|

 

[leejor]

Hopefully they can go back and see records of previous call attempts. It should also show where the call was sent, which will confirm if 3CX is registering correctly.

Did the issue start suddenly (changes made somewhere?), or has it been happening from day one?

 

[korken]

Did the issue start suddenly (changes made somewhere?), or has it been happening from day one?

It has been an issue since the beginning. It is kind of dynamic: there are time spans where communication works as usual. Until suddenly someone calls us on a mobile stating that "I can't call you on your business line - there's nothing going on until my provider just drops the call". Then I solve this by calling out through this SIP trunk and then incoming calls get processed again - for a uncertain amount of time.
Like I said - no trace of that phantom call try can be found in our logs.
Reading through thread here in this forum leads me to the suspicion that maybe something is wrong with the general STUN server/ keep alives / TTL setup - general network stuff I just can't work out where to look.
I just setup the SIP trunks in 3CX using this.

 

[YiannisH_3CX]

Then I solve this by calling out through this SIP trunk and then incoming calls get processed again - for a uncertain amount of time.

This would suggest that you are having firewall issues where incoming calls are dropped until a connection is initiated from the inside. Then the connection is re-established and kept alive until it expires again or until there hasn't been a call for a while and the firewall drops the session.