"Unless the dept is paying for their cellphones there's really no way to require them to keep them on."
Sure there is. In this day and age, plenty of employers require 24/7 ability to contact as a matter of employment. Most LEOs already have it in thier contract. I've actually had to fire people in the past for changing numbers and not informing us. Times have changed.
My guess is that someone is trying to sell a contract to install and service the system. We already have it in my city. And it was exactly that. A contractor had a buddy in both the Union and the Mayor's office.