This incident has been resolved.

All services have remained online and operational and as such, this incident will be marked as resolved.
No further impact or updates are expected.

It appears that we have been a bit hasty pointing the finger at the VoIP SIP trunks. The issue has been identified as a Microsoft Azure DNS / Routing issue.

Systems are now reporting as online and responding, however our team will continue to monitor.

We are currently investigating an interruption to SIP (phone line) registrations with multiple providers.
This is considered a major outage and priority one incident.
We will update findings as soon as more is known.