Adding an additional phone?

Sadly, this model of phone does not support adding any cordless units. It would need to have cordless phone tech already built in.

But you can still have other phones if you have additional phone jacks. If there aren't any additional phone jacks in your residence, you can purchase a telephone splitter and run a cable to your guestroom.

It's not as convenient as a cordless which is compatible with your base unit. But short of buying a new phone system with a corded base unit (assuming that's your preference) that comes with, or supports adding on cordless units, it would be your best option.