On Sun, 25 Aug 2013 11:18:34 +0100, Andy Cap wrote:

Her phone is supplied by the Post Office and it comes in through
an old
single socket. A microfilter and two-line splitter are fitted.

That'll be BT then, not the PO.

Why do you state that so categorically, do you know the OPs mother?

http://www.postoffice.co.uk/home-phone

If the line still sounds distorted, insist on an engineer's visit. If
you'd done this, they'd be hard pressed to prove it was on your
equipment and therefore apply any charge.

The fault is intermittent and in the evenings. The physical local end
is probably supplied and maintained by BT Openreach under contract
from the PO. If BT Openreach come out and don't find a fault they
will charge the PO who will no doubt pass that charge on.

I really can't think of a local end fault that could produce the
described effect. The suggestion of an overloaded link and VOIP is a
possibilty. It might be possible to force calls via another carrier
(via a prefix number) and see if those are similarly affected.

Getting this sort of intermittent thing sorted out can be a real
nightmare. Had a noisey line once BT couldn't find it and kept saying
"no fault found" until I managed to get a call to faults when the
crackles were present...

--
Cheers
Dave.