Good Morning David,
Thank you for your speedy reply.
I tried the handsets in the Master Socket, which does not have any extensions; we use wireless handsets from a base station connected to the master socket. I also tested with a normal non-wireless handset which works OK in our other VM Business telephone line (also no extensions).
FYI: We recently received new neighbours and saw telephone engineers working inside the trap outside our house over several non-consecutive days.
With kind regards, Darren.