Sorry, I replied to kasg before I sw your reply to me .
As he says, there is no Plusnet network involved on your line then, with it being PSTN. You connect to BT Wholesale kit in the exchange and from their kit it goes to the MDF.
Your Sky friend may be confused because their LLU phone lines are handled differently from bog standard BT (Openreach) Wholesale Line Rental (WLR). I'm afraid I don't know the detail.
Once again, I could have been clearer. Sorry.
Calling from some networks (Three, PlusNet) to my line, there's a problem which starts during the ringing tone. Calling from other networks (Virgin Media, THUS/Vodafone, and BT) there is no problem. The TalkTalk connection has been tested from Reading. The Three problem has been tested from Reading and London. My PlusNet service is in London.
When I say "where the call comes in to the PlusNet network", I understand that's the BT network, but I have no contractual relationship with BT. I should have said "where the call comes in to the network that PlusNet use".
The long and the short of this is, as far as PlusNet/OpenReach are concerned, my exchange line (PSTN, and not FTTC) checks out OK, so they want to come to my premises to check my equipment. This seems to me (and to my Sky friend) to be a complete waste of time. I can demonstrate the problem to the engineer, when I dial my line from a Three mobile, but s/he might think that's a Three fault. I cannot demonstrate the problem when dialling in from TalkTalk without taking the engineer with me to Reading
Of course, this could indeed be a fault on the TalkTalk network AND the identical fault on the Three network, but as it started at the same time, it seems unlikely. My guess would be that where the TalkTalk and Three networks connect to the PlusNet (aka BT) network, there's packet loss, resulting in a deafening crackle for the calling party.
I seem to have reached a stand-off with PlusNet. They cannot progress the fault, until I have booked two days off work, so I can give them the choice of three slots for OpenReach to visit. And I don't believe an OpenReach visit will do anything other than (a) give my line a clean bill of health, and (b) cost PlusNet a "false call out" fee from OpenReach.