If a number is removed from an employee in Microsoft Azure AD, is it also removed from the employee in the PBX?

Yes, if the number is external. If it's not external, you must manually remove the number in the PBX afterward or order the number to be terminated.

If the number is external, it is automatically removed in the PBX when removed in Microsoft Azure AD.

If the number is not external, it is not automatically removed in the PBX.

In this case, one must access the PBX and remove the number from the employee if the intention is to retain the number.

If you do not wish to retain the number, it must be ordered to be terminated.