Also I could save them a bunch of money getting rid of services they don't use, like moving their landlines to VOIP.
This is true, but it's not just that. How many useless cable TV packages are people paying for, on top of Netflix, Hulu, and tons of other streaming services?
VoIP doesn't necessarily require your router to be up.
For example, if you subscribe to Verizon FiOS voice, the technician will disconnect your copper phone lines and connect them to VoIP termination on your ONT.