This is a very common issue. Here is the best way to troubleshoot the issue:
Step 1: (most common)
Check: Make sure the number you are trying to call is assigned to the User, Group, Conference or Auto-attendant that you want. If the phone number is not assigned it will not work. The best way to check this is to check the list screens for Users, Groups and Auto-attendants and see if the number is listed.
User Number Assignment:
Group Number Assignment:
Auto-attendant Number Assignment:
Fix: Assign the number to a User, Group, Conference or Auto-attendant.
Step 2: (also common)
Check: Make sure the entity your number is pointed to has an extension assigned. If it does not have an extension it will not accept calls.
Fix: Assign an extension to a User, Group or Auto-attendant.
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