To answer original question, I work at a U.S. call center in Costa Rica, and I am an English teacher. It is my experience that Americans prefer to talk to Americans when getting help over the phone for problems. They don't like having to fight to understand people's accent.
I've seen call centers fire people if their accent wasn't American enough. I even saw a British person get fired. (Sad but true)
I am an American, and I get asked all the time here how to get an American accent. I usually refer them to this
excellent product http://tinyurl.com/bg2ustif they want to learn in their own home.