Area code and connection cannot be identified in &1

What causes this issue?

The telephone number &V1& is not structured according to one of the
following formats: 01234/56789
(01234)56789
01234-56789.
Therefore the individual parts of the number cannot be recognized.

System Response

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

How to fix this error?

Please specify the number in one of the above formats.

Error message extract from SAP system. Copyright SAP SE.