When you remove the faceplate do you have dial tone on your extensions? If they are dead, your external line is causing the problem. Plug a corded phone into the Test socket and dial 1 digit, can you hear any noise? If you can report it to your phone rental provider as a noisy voice line. Do not mention broadband, unless your rental is with Sky.
If you have the DG834GT as your profile says, go here and take your Detailed stats after running for a few hours, then post them here. -
Mel's Notes: Dg834gt-1skuk