It's interesting that you should cite that case. The discussion recently
came up on the URI list as to whether there should be a single "E.164"
URL type for all phone numbers, or whether there should be separate URL
types for voice, fax, etc.
The conclusion was that they had to be separate, because the user
interfaces for the handling of fax and phone needed to be different,
and also because in some cases (e.g. ISDN) the call setup actually
needed to know which was being used before the call was placed.
The http/ipp argument seems very similar to me, with a similar conclusion.
Keith