Hi
the lines are co lines.
Further update is telecoms company came out and swapped the pair for the line with another and I tried it. The same noise was there and he said that proved the ports were fine and the line was faulty.
Had the line retested whilst using it and still no fault so on a hunch I asked BT to call me on another line and test whilst line clear and there was a fault. An engineer came out very quickly and could not find fault on pair so left it disconnected from system with just his test handset on it, no fault. Reconnected to system and tried test again and fault appeared again. He suggested that it is internal wiring.
The fault was dis or ndis and I did notice on one call I made after hanging up the line flashed green, I picked it up again and the BT woman was saying please hang up.