Service has been stable for around an hour.

There will be no further updates on this issue.

An issue has been found within a third party network provider that was impacting the BT SDIN network.

Steps have been taken to stabilise the call routing within their network. Investigations are ongoing to ensure network stability.

BT are investigating an issue that is leading to a degradation of service to several CPs.  A technical bridge has been established with a view to understanding the root cause.  This has the highest priority within the business and has the full focus of all relevant technical and design communities.
This will be affecting all 24 seven services depending on routing set up.
Expect intermittent connecting and call failure issues until this is resolved.
Call re routing will ensure most calls are re-attempted but there is still a high chance of call failures.