As long as the phone is tri-band should work in NY never had no trouble when ive been there,ive always on pay as you go,if you got a contract may be best to contact provider and as them about charges i know pay as you go its very expensive to use.

so if my friend was in new york at the same time as me could i call him on his mobile?

Yes you can, if as yelzman said they are both tri-band.

Yes you can,depends which service provider you with you might need to change the number slightly in your address book,its best to check on your service provider website about use in usa i think for an internal call in the usa a few years ago i had to change the first 0 in address book to +1 then number without 0 etc.if i was you be careful of use through it is very expensive to do,best way i would do it is find out where he is staying and ring his hotel room from hotel room at local rate...