Try a hardwire phone in the master socket first and use that for a while as you may have a line fault, dial 1470 17070 and listen to the options and look for quiet line test. Also use the net for a BT line test, if your hope is fed by a dropwire (a wire from pole to your home) they pack up so easy.
Is your phone a DECT phone ? as analogue can suffer from all kinds of interference.
O yes.... This noise more likely to be the line but is it worse after rain ? Would show earth fault, a battery fault your line in contact with another in cable outside, a rectified loop could even be a phone problem also other things cause the loop so disconnect all extensions and test only from the master socket. HR fault (high resistance) a wire is about to snap! main conductor outside has gone thin.
John (ex BT, Highways, ADSL, Mega stream, PSTN, ISDN
May 2011
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Try fixfault.com to fix corded phones.