We are aware of call failures across all UK mobile networks currently. We've investigated and the issue is apparent across all 3 of our inbound carriers (BT, Magrathea, Nexbridge), and seems to be happening intermittently with all UK mobile networks (we've tested EE, Three, O2 and Vodafone).
We can see that when numbers are redialled, the calls are going through occasionally.
Unfortunately as this issue resides outside of our network infrastructure, and is happening with multiple routing carriers, we are at the mercy of the mobile networks to fix this.
Due to the unprecedented demand on networks at this time due to home working, it is likely there will be further issues with both internet and telephone networks.
Please keep an eye on https://downdetector.co.uk/ to see the latest networks experiencing issues.