SAP Message BCOMEC035 - The number to be dialed is invalid

Diagnosis
You wanted to initiate a call to the number &V1&. This number is
invalid.
Further information on the external software component:
V3

Procedure for System Administration
The error can occur in 2 different places:
1. In the connected gateway
2. In the underlying CTI software
In this case the error occurred in (&V4&).
The return code for the underlying CTI software is: &V2&