[dark-star]

I guess if you are a phone company and want to track which phone numbers are registered to which client, you don't really care if the number goes to a phone or fax machine?

[Kwpolska]

You probably wouldn’t store any of "tel:", "fax:", and "urn:msisdn:" in a database, because the column name “msisdn” is enough. Those would make sense only for data exchange, primarily links on websites.