What do you mean? There are allophones of /t/, yes...
At the end of a word (if nothing follows), /t/ can be pronounced in three different ways, as far as I know:
1) Released: you can hear the /t/, which is sometimes aspirated to some extent
2) Not released: you can't hear the /t/, but the tongue touches the roof of the mouth anyway.
3) Glottal stop: you can't hear the /t/, which becomes pretty much of a glottal stop (and so the tip of your tongue doesn't even move)
I get the impression #2 is the usual one in American English, #1 is only used occasionally, or when speaking carefully (or frequently in posh RP), and #3 is common in several British accents and some American accents (maybe African American Vernacular English).
Just my non-native guess. 