I have just noticed that there is interference in the form of hum on my phone when plugged into the upstairs extension. I have tried the phone on the master socket and there is no interference. My router is connected to the end of the extension cable and there are no problems generally with phonecalls, getting on line or with speeds. Could the fault be with the older type cable which carries the extension picking up interference? If so is there a type of cable recommended for extensions?
Problem solved! I replaced the old (including breaks/joins) solid cored cable with a continuous length of braided-core high-speed broadband cable. As it had plugs fitted I chopped these off. One end had to be tinned with solder to push-fit into the master socket terminals. Fortunately the extension socket had screw terminals.