If such are compatible I find that placing the new handset in the master base station pairs up the new handsets. If this fails, then the handsets are incompatible.
I have a five handset CS6129. I've lost one handset and broken another. I was able to buy two new 6129 handsets. I followed the manual instructions to connect with the new ones, but it won't connect. Is there a way to connect new handsets to an existing VTech system?