I work for a similar company in a different field, and I would bet money that mtnbkr is 100% correct. Once a plan gets phased out, the rate codes for it disappear from the system--they stay on current accounts but can't be swapped, added, or kept on a current account that wants to change anything else. No one can restore them below a call center director, and even then they're not allowed to. You can expect to spend a very long time on the phone talking to a lot of people just to be told no every step of the way.
Calling in isn't futile though. If they work anything like we do, you'll eventually get offered the best they can give you, which my guess would be is a free entry level digital phone. I'd give up on getting your numbers swapped and keeping the same physical phones, as it just isn't going to happen.
And by the way, don't waste your time threatening anyone below an assistant supervisor or supervisor with cancelling your service. I promise the drones do not give a flying you-know-what if you cancel, and they are not rewarded at all for making a customer happy.