Specify telephone number and telephony server

What causes this issue?

A telephone number and a telephony server must be assigned for each
work center.

System Response

Support for inbound and outbound calls cannot be activated on this work
center. Activation flags may have been deleted.

How to fix this error?

Enter the extension number for the work center and select a server
using possible entries help.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.