“What Expansion Handset is compatible with Panasonic KX-TG7644 Cordless Phone Unit?? Bought a Panasonic KX-TG7644 Cordless phone that came with 4 handsets -- for base plus 3 remotes. Need an additional remote set including base plus handset. Information with phone says Additional Handset is model number KX-TGA660 but your website says this model number is compatible with KX-TG666xx phones and DOES NOT specify compatible with KX-TG7644. Please clarify about compatibility”