If this schema is fixed as part of the specifications, then adding any
reference other than the registered version (document type) is not
needed nor desired. This http link is offering a mechanism for ad hoc
modifications or extensions. It is not, by any means, for the
programmer's convenience.
You're quite wrong about this.
The schema used in the URI isn't important. The only criteria used to
choose the existing Caller ID namespace was that it be short.
But the fact that there is a URI in the document, which names the XML
namespace in which the elements etc are found, is quite critical.