I've no idea
why it happens but I've twice had problems with my phone line that only affected my phone (with my router being unaffected).
Unplug everything else using your phone line (e.g. your router) and plug your phone into the master socket (not an extension) to see if the problem persists. If it does, and your main socket has a horizontal line across the middle of it, unscrew the bottom half
https://btbusiness.custhelp.com/rnt/rnw/img/enduser/wallsocket.gif
and plug your phone into the test socket which you've just revealed:
https://btbusiness.custhelp.com/rnt/rnw/img/enduser/opensocket.gif
If the problem persists then it's very likely to be a line fault (as I've had twice) but, before contacting your phone provider, borrow another phone from someone and try that in the test socket as well. If that also can't connect to the line properly then it's DEFINITELY a line fault and you can contact your phone provider without having to worry about the repeated warnings from BT OpenReach that they'll send you a massive bill if you call them out needlessly.