I have an 816 w/a 160 (door bell) card. The door bell rings on my KX-T7433 phones but not my KX-T7420 phones. I tried switching the phones to make sure the extensions were programmed correctly, and they were. What else can it be? These are two 7433 which work anywhere and three 7420 which light up but do not ring(only for door bell) anywhere. Thanks in advance for your help.