I have this problem too. Bit annoying! Especially when itís a sales/ppi/microsoft scam phone call.
Had many engineers out, plus replaced the home hub, replaced all the filters, replaced the master socket, none of it fixed the issue.
Suspect itís the cabling back to the cabinet, dodgy and/or wet joints.
BT ADSL customer getting 4.2 Mbps (0.7 Mbps up) on a new road / new build development
(Was circa 1.6 Mbps for the first 4 years)