My mother's phone has developed a habit of deteriorating in quality in
the evenings. It seems to get markedly quieter and has an odd "talking
from inside a cardboard box" quality. This seems to happen quite often
but getting a proper analysis of exactly which days and for how long
just ain't going to happen.

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.

She has two phones - one wired, one DECT - both seem similarly affected.
Both work absolutely fine at every other time. And all the wires have
been taken apart and re-assembled more than once without changing the
issue - including changing the microfilter and splitter.

She (or someone) has contacted the PO and they seem to have run some
tests and declared the line to be fine. I strongly suspect that any
tests were run during the day so we agree they'd be likely to be fine -
the issue only happens in the evenings.

At the moment, I am hypothesising that the issue is on the line rather
than an actual phone.

What DIY testing is possible? Are there any diagnostic numbers available?

It is a fair old drive to get there so the ideal would be something I
can explain to her. If it is simple enough she can do the test when the
line is bad and, hopefully, identify the issue. But if I need to go
over, so be it.

--
Rod