I've got a customer that has SIP trunks on a IPO 9.0 Partner edition. They are saying that they can make outgoing calls but can't receive incoming calls. When you call their main number you hear the message that “you have reached a nonworking number. Announcement 14 switch 18-3.”. Their SIP provider is Allied and they are saying that there’s a routing issue with the PBX. They’re getting 503 service unavailable errors to their SBC. There have been no configuration changes so says the customer. I will be comparing the original cfg file saved to my laptop to their current cfg to make sure. The customer also said that now it seems they are able to receive calls for about 30 minutes and then it goes back down. This pattern repeats itself throughout the day. My questions are has anyone experienced this issue before and where do I begin to troubleshoot? I checked alarms and there are none. Strange. Thanks for any help you can give.

Never tried SIP lines on AVAYA before.

But I think it might be a SIP trunk keep-alive issue.
every time when make outgoing call, phone system will talk with ISP SIP server. But ISP SIP server could not find phone system when keep-alive timer times out and the SIP link is not active.

You might check SIP trunks settings, or similar settings on local router/firewall.

The router likely needs to be setup to forward port 5060 to the internal address of the IPO. You'll want to restrict it so it only forwards traffic from the VoIP provider's public IP.

If you upgrade them to Essential, you would be able to set up STUN, which, depending on the provider, could solve the issue.

It seems that the URI somehow changed although the customer doesn't have manager installed and couldn't have changed it. The provider says they didn't change anything so WTH. Anyway changed it back to the provider settings and everything is up and running again. Thanks for the helpful suggestions.