panasonic easa-phone handset doesn't communicate with base

[pstock]

I love having a speaker phone for those annoying and often extended On
Hold waits for customer service or tech support. A few years ago I
picked up a 2nd hand basic panasonic easa-phone cordless phone and base
model KX-T3910H (call it Unit A) and have had good success with it.

Saw the same model (call it Unit B) in a thrift shop the other day and
so for $4 picked it up. Usually these older cordless phones just need
their NiCad batteries replaced ( on this one there the battery had been
removed). I recharged the new battery in HandSet B overnight as
instructed.

Problem. Using Handset B with Base A I get a dial tone on both the
hands-free base (A) and the handset (B). Both phoning "vehicles" work
fine.

Swapping the phone line and charger over to Base B, however, Base B
works fine on handsfree but I cannot get a dial tone via Base B on
either Handset A or Handset B.

Now, maybe THIS is why the previous owner got rid of it.
Alas, I am not one to dig into the electronic guts of one of these
units to troubleshoot. But can anyone shed any light on why something
so normally foolproof would just stop communicating??

thanks

Peter