I'm not sure what that link is intended to prove. Americans are typically quite tolerant of accents in English because we encounter so many of them in daily life. There are lots of people with Indian accents at my workplace, for example.

I make an effort and someone has to have quite bad speaking ability before I give up on them. This happens with Indian call centers, but not all the time. The bigger problem is that the call center staff, which is almost always outsourced to some contractor that handles many diffeent companies, are given no authority or training. So even if you understand them, they can't or don't want to help you and are often limited to reading from a script.

Being told, "sorry, I can't refer you onwards to a domestic service agent," and then being referred to one when I request a supervisor is a perfect example of that type of problem.