Most if not all modems have an in and out port on the back so you can plug a phone through it. But most likely in this situation, the house has a split in the phone line so that you could have multiple phones in different rooms.
So the op was in the office with the computer and the mom was in the kitchen trying to call someone else.
Wait, you didn’t have to manually unplug the modem and plug in the telephone to use it?
Most if not all modems have an in and out port on the back so you can plug a phone through it. But most likely in this situation, the house has a split in the phone line so that you could have multiple phones in different rooms.
So the op was in the office with the computer and the mom was in the kitchen trying to call someone else.