Hi Mik, 

From your description it sounds like their might be a carrier related issue either on the RingCentral side or with an intermediary or terminating carrier. Basically there might be a road block or traffic jam and traffic might need to be detoured. 

The best course of action will be to Open a RingCentral Tech Support Case

For these types of issues we would want to see 2-4 detailed call examples (template below) from the last 24 hours, we can investigate those calls and see where a re-route or other fix may be needed: 

Call Example 1:
Number (From):
Number (To):
Date: 2020-03-16
Time+Timezone: 
Result: Heard announcement  that the call cannot be completed at this time.

Call Example 2:
Number (From):
Number (To):
Date: 2020-03-16
Time+Timezone: 
Result: Heard announcement  that the call cannot be completed at this time.

Call Example 3:
Number (From):
Number (To):
Date: 2020-03-16
Time+Timezone: 
Result: Heard announcement  that the call cannot be completed at this time.

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Why?  Possibly because everyone that can suddenly started working from home.  It is putting a huge strain on every provider and every carrier.

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