Site '&1' cannot be deleted because subobjects exist

What causes this issue?

You tried to delete the site '&V1&'.

System Response

This is not possible because sub-objects still exist for this site.
Possible sub-objects are telephony servers, number/user groups or
exception numbers that have been assigned under 'all other numbers'.

How to fix this error?

First delete the existing sub-objects.

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.