I must be doing something very stupid with my new phone because obviously the mute worked fine on my old phone. And both are Poco's (Xiaomi sub-brand) so it's likely I've done something silly. I'll press a few more buttons next time I'm working and see if I can fix it. Thanks all!
And, RC, before you ask, I'm pretty sure it wasn't a (im)migrant that sneaked into my house one night and changed the setting.