Our SMS gateway is currently unable to relay via a specific carrier. The problem looks to be at that carrier's end. We're looking to get this fixed as soon as possible. We are also looking into why a problem with a single carrier could affect SMS in this way, and we are looking into making changes to improve resilience.
We've improved resilience now, which has helped a lot. Investigation of the original underlying problem is still ongoing.
The carrier in question is aware of the problem and will be fixing their side soon. In the mean time, our improved failover appears to be handling the problem correctly and this problem should no longer be affecting customers.