A phone number value type should actually be strictly validated using a user-defined string - and the default should be a 12-character string like this: “0nn nnn nnnn”
Some country databases might use different number systems, and where database content spans multiple countries the typical format would be “+cc nn nnn nnnn” (with cc being the country code).
Although some fixed phone numbers will have extensions, those are from what I’ve seen less and less common (and can be handled using a standard 3 or 4-character text value type)