Inbound calls not being receiving until outbound call is made

[DanMcL]

We're using Gamma as our SIP Trunk provider on a self-hosted 3CX platform running the latest version.
We can't receive incoming calls until an outbound call is made on that trunk. This then lasts for a few hours before dropping out again and needing a new outbound call made.
It's behind an Edgerouter and is natted with all ports open and SIP ALG is disabled, there is no IPS (that I am aware of).
The firewall status reports as Green in our 3CX console.

 

[Evolute IT]

Does the trunk stay registered even when no calls are incoming?

Also, what PBX version are you using?

 

[YiannisH_3CX]

Hello @DanMcL

It looks to me like a firewall issue. The firewall is blocking the connections until an outbound call is made which isn't blocked and the connection is active for a while or until no more calls are active and the firewall reaches the timeout. Then you need another outbound call so the firewall reopens the connection.
I guess your firewall should have a setting for this so the ports are always open.