Active just BBM on my phone?

Hey guys...I have a quick question. I have an Airtel prepaid sim from South India. I have been using it for a few years now, but then I moved to San Francisco. So I've been keeping the sim alive by charging it for like 30 bucks every 3 months or so. The sim is just lying around so I wondered if it would be possible to activate the BBM on it, and buy a Blackberry from here in the USA and use it with that? Someone told me that the BBM feature was free. Does this mean I don't need a data connection for the BBM and wont be charged roaming? I'm not really sure how it works.Thanks!