[Dave Moll]

It is also worth checking the health of the battery in the one whose transmitter (microphone) isn't working - though first try Russell's suggestion of swapping handsets first to confirm whether the problem lies with the handset or the main unit.

Do your units have buzzer units? This can be established by checking whether the plug-in unit in the right-hand lobe of the body is an enclosed unit with two knobs on top (buzzer) or an open unit containing just an induction coil (no buzzer).

If one or both have buzzers, try pressing the button on the front right-hand side to the left of the handset plug and checking whether the buzz is audible in the other handset. This will have the dual effect of checking the battery health of the initiating set and that the receiver in the other set is operational. If only one has a buzzer, then try swapping the handsets and repeating to check the second receiver.

edit: (presumably owing to need for moderator approval on initial posts) Ben's second post hadn't appeared when I made my post, so forget battery checks. It is still worth checking whether the problem lies with the transmitter or receiver.

In the absence of a buzzer, the receiver can be checked by connecting a low-voltage battery across the inner two prongs of the handset plug - with the handset unplugged from the main unit.