Graham wrote
Harry Bloomfield wrote

Waiting to get through to Plusnet's support, they play music
whilst waiting. If their is any noise in the room, the music mutes,

Never seen that and I have been on hold quite a bit
lately due to changing various service providers, in
fact due to changing all but the local council with
some glitches with every single one.

however they also break into the music to make a regular
voice announcement - the voice announcements are NOT
affected/ NOT muted by noise in the room.

Presumably to reassure you that you are still on hold and
that the call hasn't got lost in the system or something.

One of my banks has very elegant and quiet
music on hold, but the voice announcements
spoil that by blasting thru as usual.

Best guess is that Plusnet's system listens for
noise on the line and mutes just the music.

There isnt any other way to do it unless I am missing your point completely.

I don't really know why they are doing this
but I am prepared to engage in speculation.

Particularly with the longer times on hold, I would prefer to
be able to do what I normally do and have my system mute
what I am currently listening to when they eventually come
off hold and a human is there to talk to. One of ours is notorious
for an hour on hold at times and another isnt much faster to
answer. And neither is a govt dept.

It occurs to me that their music on hold might be configured
to mute when inbound audio is heard in case a valid DTMF
tone is sent by you to their IVR, so the music does not swamp it.

Surely a decent system should have a good enough pll filter so
that the tones can be detected thru almost any background noise.

In theory this is not necessary as the TX and RX is separated out,
including in the "hybrid" circuit of your telephone, but in practice
there is always some leakage, some of it intentional (sidetone)

On a related matter I have pondered whether, when you call an office
dealing with "benefits" or insurance claims, if they record the call
while you are waiting to be put through to the department, in the
"hope" that you will conspire with someone else in the room about the
fraud you are engaged with.

If they did, you'd expect that some ****ed off employee
would have spilled the beans on them by now.