If calls are failing for the entire organization, start in the Microsoft Admin Center:

1. Check Domains for health

2. Ensure Microsoft Domestic Calling Plan is not in use

3. Ensure all of SimpleVoIP's IP addresses are whitelisted.

If calls are only failing for a particular user, start in the Teams Admin Center:

1. Check the PSTN Usage for the correct usage

2. Check the Voice Route Policy for the PSTN Usage for the correct policy

3. Check the VR and verify if the DR domain is present or not

4. Check the User’s phone number

5. Make sure the User’s phone number appears in the proper format and is the correct number that we are expecting the user to have

6. Check the User’s policy and verify if it is correct policy.

After completing those steps, if the issue persists, please open a support ticket.