This is an external dependency (a python library that needs to be installed).

In CLI, run

python -m pip install phonenumbers

then, restart your Odoo service and retry.

Details about 'pip' (and possible issues with multiple python versions being available in the same system) can be found here: https://www.odoo.com/forum/1/279063 

Documentation on installing Odoo is at https://www.odoo.com/documentation/18.0/administration/on_premise/source.html