Hahaha - very timely thread! I was talking to my daughter about this only yesterday and remembering my beloved Welsh mam who died two years ago and had a very posh phone voice. It was so funny to hear - she'd go all Hyacinth Bucket, with not a trace of a Welsh accent, when she answered the phone - absolutely hysterical.
In real life, I too have a variety of voices depending on who I'm talking to. I can range from proper Cockney geezer to modulated middle class England and all varieties in between. I also have an 'angry' voice apparently - and people know I'm pissed off about something, not because of the words I use but the tone in which I speak.
Me and the other half also have a terrible habit (which I am sure is going to get us into trouble one of these days) of slipping into the local accent when we visit other parts of the UK. It's not intentional and not meant to be rude or offensive to the locals in any way, but it's just something that we do and need to stop!!
On a fab holiday in Yorkshire we were having dinner in a gorgeous country pub and there were a group of youngsters on the next table with the broadest Yorkshire accents I have ever heard and proper 'thees' and 'thas'. It was so wonderful to listen to ... but after that when chatting to each other in the car as we toured round, we'd occasionally slip into mimicry!
Same in Cornwall last year - doing the Cornish accent. Why I have no idea. Thankfully it's just between ourselves, but I'm terrified one day we will accidentally slip into our fake local accents in front of people and cause offence!