Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble.
Unfortunately the existing phones you have are a special model designed to interface with your home pbx system.
They cannot be simply swapped out with another unit.
What you could do is bypass the pbx box, and wired a CO (incoming) line to a wall jack directly, and then connect a new cordless phone system base station to it.
This would let you make/receive calls (on 1 line only) from those cordless phones and get caller id. However, the cordless phones would no longer be part of the main system, you would't be able to use them for intercom functions, transferring, putting calls on hold, etc.