Why is my DeutschlandLAN Cloud PBX line not working?

Problem:

The line diagnostics show that the line is down due to an error. Additionally, the following message appears:

Update of capabilities failed: address lookup is ambiguous

Solution:

There is a user setting in the “Privacy” section of the Telekom portal. The option “Allow display in company directory” must be enabled here; otherwise, the line will not go online in XPhone.