No sound until I press & release "hold"

[hnp321]

Hello,

When we ring an external number, we can hear the ringing tone... but once the person we are ringing picks up their phone the sound stops (i.e. we cannot hear them) and we only get the sound back if we immediately press "hold" and the "unhold" on the phone handsets.

Any help to resolve this would be appreciated.

Thanks in advance.

Harry