what's your problem kabayan?
I believe that you're overlooking the fact that call center agents in the Philippines are taking calls mostly (if not 100%) from Americans, and the "thwang" is needed to communicate more efficiently with them.
Imagine this, a visayan talking to a tagalog in Filipino. If the Visayan can't pronounce or enunciate properly what he is saying in the "right" tagalog way then there will be difficulties understanding what he's talking about. Same is true with a Pinoy talking to an American, it's not enough that you are gramatically correct but at least can talk clearly - and im not saying 100% american accent.
Let's not look at it as "talking like an American" but "communicating more efficiently with them".