Telephone number could not be converted to canonical format

What causes this issue?

Unable to convert the telephone number to canonical format.

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?

Change the number and ensure that the area code is clearly separated
from the main number, for example, such as:
1. (01234)56789
2. 01234/56789
3. 01234.56789

Error message extract from SAP system. Copyright SAP SE.