Call2Teams - Customer
This article outlines some of the common issue with setting up Phone App and the suggested resolution.
1) Users not showing in the Add Users list
The most likley cause of this is a user or the tenant not being set to Teams Only mode.
In the Sync Now log the following type of message will be reported:
Excluded 2 users ineligible for Phone App as they are not in TeamsOnly mode for Teams Upgrade.
Review this setup article for remedial instructions on setting a user or the entire account to Teams Only mode: Phone App (Preview)
2) Calls not going through to a Teams user - '603 Call Declined' in call log
This issue is a result of the customer Microsoft 365 account not being correctly set for External Access and allowing the Phone App domain.
The solution is to allow the Phone App domain on the Microsoft 365 tenant in the external access settings, or allow all domains
Directions for setting External Access for Phone App are available in this article Phone App (Overview)
Note: it can take some time (maybe several hours) for the External Access domain settings to take effect in the Teams admin center