I can't find a category for 'phone line' so here goes:
I've just switched from Virgin, previously I always got the callers number on my phone display, now 50% of the time it just says 'Incoming Caller'.
Also when the caller goes through to my answerphone & hangs up it does not disconnect - it plays the message then records a high pitch screech as a message.
A bit of googling tells me this is due to the exchange not dropping the call, then assuming I have left my phone off the hook & sending a screech to alert me.
As said, everything worked fine on Virgin, and before that I was with TalkTalk and again everything worked, so I think it is the Now exchange equipment at fault.
I've tried to find a way of contacting them, but if it's not a TV or broadband fault there doesn't seem to be any options!
The number provided by @redchiz1 is also in this link, which directly says about phone line issues to call the number.