It's not like they really care what number I use, because they'll cheerfully assign me a completely different phone number to use with that contract obligation. And it's not like they really care which phone instrument I use, because they'll even let me swap phones online through a web interface. So all they've really got (or appear to care about) is that I promise to maintain service with them, using some phone number, and some phone, for the next year. And I've got no problem with that.
I'm not trying to avoid paying them, I'm just trying to avoid paying a termination fee for a contract that I don't want/need to terminate.