Zoom have confirmed that they are investigating the issue on their status page: 

https://status.zoom.us/incidents/wyptrc2myzy0

Inbound and outbound calls are both affected.

Please note, due to this issue, our IT Support team are currently struggling to answer calls. If you have a query, please email itsupport@york.ac.uk.

Zoom Phone has been restored for incoming and outgoing calls.

We will continue to monitor the situation.

We do not anticipate any further issues with this service.

The issues are now resolved.