2nd August 2019

Routing - Outbound Increased number of 503 Service Unavailable responses

Whilst we have still yet to receive updates from our upstream carriers as to what the cause of this issue was, we have been monitoring SIP Responses across our network and can see that this issue has subsided. We will continue to push for updates.

Older information:


Further to monitoring of this issue, we have seen a significant decrease in the number of 5XX responses being sent from our upstream carriers. We are continuing to monitor this issue as we have yet to receive confirmation that the issue has been resolved.


We have received further reports from our upstream carriers that this issue is not limited to them or the Nexbridge Network and they are also receiving a range of 5XX errors from their carriers. Upon testing with PSTN Equipment, we have also been able to replicate issues from a random set of numbers, whereby a BT Network Level announcement of "Sorry, there is a fault" is being played. Whilst there is a high number of SIP 503 responses being returned, we are still seeing traffic flow on our network, with 5XX errors currently accounting for less than 10% of our traffic. We will continue to update as information is made available. This does not affect inbound routing.