Quote Originally Posted by j4mes_bond25
I thought such an additional "t" was only a part of British English. But it seems the same goes for North American accent, as well then ???
Not to my knowledge, but North America is a big place! There are a great many dialects to go through.

As for me, I don't insert [t] in the words in our list. The one exception "in[t]stant" has to do with syllable weight. [t] is inserted syllable finally, after <n> to balance the weight of articulatory movements across the syllable boundary:

in_stant => in[t]_ stant

The onset <st> is heavy, whereas the coda <n> was too light.

By the way, does this "t" in between "n" and "s" sound a part of RP accent or is it seen as a part of "posh" English ???
I'm not 100% sure, but the source we're working from is Cambridge, so I suspect [t] is a phonetic characteristic of Standard British English, RP. Check with tdol, teh site editor here at UsingEnglish.com. Private Message him. He'll know.

Lastly, speaking of "posh" English, RP and Plummy are seen as "posh" in Britain. What is the equivalent there in North America?
Well, to my knowledge, RP finds its origins in the class-system, which is something we don't really have in North America (Psst. Our ancestors left all that behind). As for a North American term that's synonymous with "posh", I am not sure there is one. Phrases such as "speaks well", "well educated", "sounds British", "highfalutin" (grand or self-important in a pretentious way), "pretentious" come to mind.