My DECT phones regularly beep then seem to disconnect from the base unit. After about 30secs I get a two tone beep and they seem to reconnect. BT auto line fault checker reports no fault on the line. Last week my router kept disconnecting and Sky's technical help said the problem was pointing to the filters. I only have one filter on the Sky box and an ADSL nation faceplate (? proper name) on the master socket.
Could be interference from another 2.4 GHz device. Perhaps a new one on the same channel - maybe a neighbours, could be the WiFi on a Broadband Router, could be a Bluetooth enabled mobile phone. So many things use this part of the spectrum that it's difficult to predict what the problem is. Suspect everything!