Sms Quarter

how do sms internet websites make money off of wireless providers?

hi,
i want to know how do the companies who offer sms through internet make money off of various wireless providers?
1. do they register people and send bill to wireless providers or
2. they have arranged with each providers or
3. is there a place (head quarter) that they have arranged for many providers? and getting paid
for #2 and 3, how do they contact them?

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U can refer to

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2008 AQHA Congress NY 14-18 SMS