We've ownd an AT&T Model 5870 Base with one Model 5800 Expansion Handset for several years. We recently bought a newer Model 5800 Expansion Handset at a garage sale but can't figure out out to link it to the base unit. The instructions list it as compatible with the 5870 Base but the registration procedure requires a 15 digit base ID, but the base ID on our 5870 Base only has 14 digits. I've tried inserting a zero in various places but nothing has worked. Does anyone know how to link a handset that calls for a 15 digit base ID to a base with a 14 digit ID??