I would like to send a messages to all iphones or blackberries or androids that are using my local wifi. Whether it be a bbm or a flash sms etc.... I am even willing to pay for texting etc....

Can I do so without knowing their number? Can I determine their number from their information (ie: mac address and or phone id) being sent a request for phone information?

I appreciate any help. I have a feeling like it is not possible for many reasons, but I am curious....