[upofadown]

Most other chat services use the phone number as a unique identifier that aids discovery of other people the users know. So using it just to drive up the cost of sending spam is kind of a waste. If the phone number was used as an identifier then most would be happy to just ignore attempts to start a discussion from people they don't presently have in their phone book.

In general, there are better approaches to avoiding spam in an IM system...