Hello. You can only connect another phone of the same type to the base station.
Does the paperwork for kx-tgd220c say you can add a kxtga106c as an extension?
Unfortunately, the kxtga106c is only compatible with the Panasonic Digital Cordless Phone: KX-TG1061
It's not compatible with the kx-tgd220c.
Are you seeing something different in your paperwork?