I do managed security services, and we use modems to manage routers, firewalls, intrusion detection, and similar devices. It's not the primary management mechanism, but you need to be able to talk to the console when the Internet connection's not working, or when your box wasn't staged correctly so it doesn't have the right IP address, or when the box has gotten too hosed to do anything other than power-cycle it. At more permanent locations we'll install networks of terminal servers so we can get serial connections to the console ports without dealing with POTS lines, but usually at customer sites it's dial. And getting customers to connect the "line" side of a modem to the wall and the "phone" side to the router's console port has gotten a lot tougher the last few years.
I doubt it's still done this way, but for a long time the 4ESS phone switches had 56 kbps X.25 satellite connections as backup management. 56k was still tolerably fast, X.25 wasn't obsolete yet, and satellite was a way to get a connection when backhoes or floods had taken out land-based connections.