Switching landline from Verizon to Xfinity

Long story short, I'm switching a family members landline service over from Verizon (not Fios) to Xfinity. They have been getting their internet and cable through Comcast for years, so it just makes sense to bundle everything up with them. I went online and completed the landline number transfer through Xfinity's website and added the home phone service to their plan. They are sending out a new xFi Gateway which should arrive in a few days. However, Comcast customer service said that their home phone service will work with her current modem, so the new one being sent isn't necessary, it's just a courtesy upgrade so they can access faster internet speeds.

My question is, it's an old house, and currently the back of the modem doesn't have a modular base cord (phone cord) plugged in. It's just a power adapter, coaxial, and Ethernet that's connected. So, I'm assuming, I'm going to need to figure out how and where from to connect the phone line to the back of the modem so the new phone service can be accessed since it will have to run through the modem now. At my residence there's a Y-shaped connector next to the fuse panel in the basement. From it a phone line runs out and along the base of the floor where it connects via jack to the back of my xFi Gateway. Very simple. The house where I'm connecting this new service has no such Y-connector anywhere to be found. I'm just thinking ahead so they aren't without a house phone should they need to schedule a tech to come out to look at things. What's the most likely scenario here? How can I make this work with the current setup? Thanks!