Hi all: any opinions?

I installed two simple wireless bells for a neighbour the other week
(this model in fact: http://tinyurl.com/oly67qv - made by Byron)
(battery bellpush, chime plugs into mains).

She needs one at the front door and one at the back door, each playing a
different melody so that she knows where the caller is.

The bells worked just as we hoped when I installed them, but this week
the back door stopped working: push the button: nothing. The front door
worked as before.

I expected that it was a battery malfunction or something similar, but
No it wasn't.

I unplugged the one I suspected to be at fault (the two sit side by side
in a double socket).

Found nothing wrong, plugged it in again.

Then, BOTH bellpushes operated BOTH bells, with an identical chime.


Following instructions that came with the bell, I re-set the back door,
i.e. made it re-learn its pairing and all now seems to be working as
before.

HOWEVER: I don't understand what happened, and therefore how it might be
avoided in futu any suggestions?

Thanks
John