Are ALL the phones crackling? Have you tried unplugging all but one, and putting that into the Master socket? If that solves the crackling, you can try plugging back in the other bits of equipment one at a time, and seeing if one of them causes the crackling. Also, modern master sockets have a "test socket" located under the fascia which connects directly into the BT line. If you have one of these Master sockets, you can simply unscrew the front to reveal this hidden socket. Plugging equipment into this shuts off all extensions and other wiring etc. If there is no crackle then, then you know it's either some of your equipment or wiring. If there is crackling, it's a problem with your line, and therefore BT's problem not yours, so no charge to fix.