Deregister handset for panasonic kxtg5438s

Someone gave me two handsets for their panasonic kx-tg5438s but I don't have the base station to facilitate deregistering. I have a kx-tg5632 panasonic base station that I'm trying to register these new handsets to which supports up to four handsets. I found online the menu#131# code but that doesn't seem to work for the 38s handsets. Is there a different code or different way as accessing the registration screen on the handset? Please advise. Thanks.