Anyone familiar with "603 Decline" SIP errors?

Our IPedge was working fine then suddenly stopped accepting inbound calls & VM went to "camp-on". Packet inspection showed constant "603 Decline" SIP errors for inbound calls & VM ports. Seemed like something was consuming all available ports. Oddly, when the entire system was rebooted everything worked fine for a few minutes but then quickly failed with 603 errors.

Seems like a PBX problem but we've had similar inbound call problems before & it's always due to our carrier. So I opened a ticket with them & it turns out they'd worked on a circuit that we are part of that morning. They said testing revealed no problems on their end. But when I got to work this morning, everything is working fine.

Has anyone else experienced this or know what's going on?

TIA!