at&t phone and sim card help?

[pats]

i have a new at&t sim card i got a phone from china it has no sim card and i put my sim card in i am in india now so my sim has no signal on my at&t phone and i put it in china phone and it says emergency and might work in america. will at&t let me use it? will it work? or should i just not tell them and use it if it works? my main question is will it work because it works here in india