If your phone company tells you that you’re required by law to keep on using the service that you’re paying them for, then ask someone else. Or tell them that you’re going to stop paying them, but that they’re required by law to keep giving you basic service.
I picked up one of these cellular gateways when I was going to drop my landline, which I think is literally what the OP is asking about. I think Amazon sells them now. I ended up keeping the landline so I’ve only done rudimentary testing with this thing, but it seems to work. The BT model lets you connect one standard telephone to the gateway, so if the phone is a cordless base station with multiple handsets it does the same thing as Panasonic’s Link2Cell, but with any telephone. The BTTN model can also connect to the home’s phone wiring, so it enables all the phones in the house. (An important caution when connecting any device like this to the home wiring system is to disconnect the system from the telco service line, which continues to supply voltage even when service is cancelled and can damage devices like this.)
I can’t attest to how reliable it may be in the long term. The two disadvantages I noted are (a) the Bluetooth range is very limited, so wherever you normally leave your cell phone to charge has to be very close to the device for it to work reliably, and (b) call display isn’t fully functional – IIRC, it picks up the incoming number from the cell phone’s call display, but not the name. I think years ago there were also complaints that it didn’t work with some cell phones. Caveat emptor, YMMV, etc.
The one my mom got, the box itself connects to the cell network, without having to go through a separate cell phone. So you just need to make sure to put the box someplace with good cell reception (and, of course, within cord’s reach of one of your phone outlets).