When having a conversation the volume reduces until neither party can hear?

When using the handset in our living room - about 6 meters away from the docking staion in the hall, the volume gradually reduces so that neither party can hear and the call has to be terminated. The battery seems to be ok in that the indicator shows it to be fully charges.

We have two handsets so are using the other one to see if we have the same symptoms. Meanwhile any views/advice will be appreciated. By the way it is a NTL VS 2001DECT phone