It is unlikely that a page recipient will have more than one service
provider, so I'd guess that the kind of question you are raising
won't be a real problem for paging. But there certainly may/will be
other uses for tpc.int (e.g., the original fax printing) in which
multiple providers will be available. (In fact, there already are.)
The current scheme has the major benefit of not requiring that the sender
know anything special about any given recipient, other than the standard
core piece of information, namely the telephone number. You raise
the concern that this very approach PREVENTS the sender from making a
choice, if they want to.
I did some checking and it turns out that the assumption isn't correct.
The DNS returns the list of ALL providers. If you don't care, choose the
first. If you do care, look deeper at the list.