If you call SQ's toll free U.S. number, it goes to the call center in India. From my personal experience, one will get a local SQ agent, instead of at an outsourced center if one is a krisflyer member and enters her/his krisflyer number.

Again, from personal experience, talking to agents in India, were, at times, very challenging (when BTC and at the same time, getting a seat assignment changed amongst other things). It's as though they did not fully understand English. Some were downright rude. That's when I would ask to speak to a supervisor. I've been made to wait up to 20 mins for one. But hey, I am a very patient person. I will make it a point to wait for a supervisor, to put forth my point, if I feel I've been treated rudely. No reason for bad manners.

I've never had problems with understanding the english of local agents (singapore call center), and vice versa. The problems I ran into were largely agents being "obtuse", rude or just plain unhelpful. Some of their spoken english may be accented, but I find them to be perfectly understandable and competent. I am pretty certain that Singapore's local lingo is neither teochew of fukienese (however it's spelt).
Singaporeans, or at least the ones I know or the ones I've come into contact with, feel very comfortable speaking singlish to one another. They are too, perfectly capable and competent with "proper", understandable english, albeit accented. Then again, there's nothing wrong with accented english, other than the popular (in my opinion) American or British accented ones.

Some of the outsourced call centers when calling US carriers or some other matters, I have to admit that it's like pulling teeth! May as well tranquilize me! UA has call centers in India and the Philippines (when I called). India call centers agents - same problem with english, like SQ's agents (some). Philippines call center - nerve-wrecking rudeness! (just my personal experience).

When it's outsourced, i.e. India, it's not just SQ agents struggling. It's the "struggle" of any airline /business agents (my personal observation and interaction). And please pardon my grammatical errors. Just got off a long-haul flight.