I would doubt that the additional revenue would pay for the additional costs. One opportunity cost that this would raise is a security concern, at least for analog phones: two appearances of the "same" phone indicated fraud.

Development costs could be significant as carriers try to deal with "partial roaming" calls: two phones could simultaneously ring in, say, Dallas and Sydney. Ringback is supposed to be provided by the far-end (i.e. "the" ringing phone), with the voice channel set up prior to answering: choosing which target phone is the primary, and managing timing issues among the various targets would raise plenty of development problems.

If all you want is the "ring multiple phones" service as you already have it, then, well, don't you already have it?