Well, they do have different accents (Bury sound more like a Lancs mill town to me) but from a non-local perspective they don't sound hugely different. Certainly nothing like the difference between "Scouse" and "Manc". Its also got a lot do to with class.
I agree about Bury/Altrincham not being Manchester/Liverpool. However, to me most people in Altrincham talk with the same sort of accent I hear in Knutsford. Manchester and Bury accents sound like different accents.
Yes class does come in it. The poshest sounding people I've ever heard are from Hale near Altrincham. However, all towns have different social class, it's just some have bigger gaps between classes than others.
Going back to Altrincham/Manchester/Bury. If there was another Metrolink type system that did Doncaster-Sheffield-Chesterfield, I would say there would be less variation in accent on that route.
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That is true, but I think there is more than a grian of truth in the idea that some regional accents are considered better for certain purposes!
Unfortunately call centres do always seem to go to the cheapest option, rather than the best, which is why so many have gone to India.
I imagine county wide that a call centre based in Bristol would be more popular with customers than one based in Glasgow. However, if the call centre in question is Scottish Power's (who took over MANWEB - the MANW bit standing for Merseyside and North Wales) then I imagine a Wrexham call centre would be a better option due to the accent being similar to a large chunk of their primary market.
Jason Manford once did a joke about phoning a call centre and the person answering having a Wythenshawe accent, so he said he decided not to buy over the phone.