When a phone number is added to the Basic information > Personal contact number field in Configuration Manager user profile, it cannot be used as a forwarding phone number in Call forwarding rules section of the user profile.
When a phone number is configured in Personal contact number and saved, the number is bound to the user's assigned extension, and appears as the extension in external forwarding.
To resolve this issue, do the following:
- Use a different phone number than the one which added to the Personal contact number field.
- Or configure the external number forwarding first, then add the number as the Personal contact number.