there are two lines of thought.
sensible and socially responsible:
why disable an existing and working system that has advantages over the new system? at the very least, make outgoing calls free for emergency purposes.
shortsighted asshole capitalist:
it costs money to maintain, so just unplug it as soon as contractually possible. when they somehow manage to call your support staff, tell them that they will need to upgrade to your cable internet + VOIP service and transfer them to sales. if they are rural and thus too far out to actually make a profit from installing new cabling, tell them they cant get it and politely hang up. be sure to use your hired company that keeps track of online forums and rating sites to blast anyone that is upset.
which do you think your telecom is going to fall under?