on extensions - these were installed by an electrician when their house was renovated 2½ years ago
Speaking from experience, the "trades" should stick to, and only be asked to do what they are best at. An electrician should be just that, they shouldn't be trying to install voice or data cabling because you have to treat the system and its cables with a bit more delicacy than you can drag some multi-mm core electrical cable around.
Simple fact of the matter that if there is crackling on the line, and you have tried a known-good wired phone at the test socket master socket, then its BTs fault and they need to clear it.