We're raising a fault with one of our VoIP carriers where some incoming calls are ending after a few seconds. This isn't all calls, it may be specific to calls from some mobile numbers - we're still investigating.
We have a reproducible example of a failing call that we can use with the carrier to further diagnose. Investigations are still underway.
Whilst investigations are under way, we'll explain the problem a bit more. Some customers are having incoming calls end after a few seconds.
The cause of this is that the SIP messages between the incoming carrier and us are not being generated correctly.
An incoming call is made via a series of SIP messages between the two parties, typically we'll be sent an INVITE with details of the call, and then we'll reply to say we're TRYING and then RINGING the other end. When the call is answered by the customer we'll send an OK message and the carrier will acknowledge with an ACK message.
The problem we have, is that the ACK message coming back from the carrier is not matching the call properly (the format of the phone numbers involved are formatted in different ways), and so after a few seconds the call is deemed to have failed and it will be ended. This is not as per the SIP specification (RFC 3261 section 220.127.116.11).
The problem is intermittent and not affecting all numbers, but now we have a suitable reproducible example, the carrier is able to enable their diagnostics and are investigating further.
The carrier have logged an example call are are investigating.
The carrier are escalating the issue at their side and will be getting back to us.
We're still waiting to hear back from the carrier.
Latest from the carrier is that it's still with their technical team.
A change has been put in that at the carrier end that has fixed this problem.
This issue only related to ported numbers on one of our carriers, and even then not all ported numbers as it depended on the route calls took getting in to the carrier's network. The carrier fixed the problem with the SIP messages and calls have been fine since 21:45 this evening. Ported numbers we have with our other carriers were unaffected.