GSM gateways in the US?!?

$subj says it pretty much all :) Does anybody use anything like this in the US? The $1 mil/yr costs of our cellular telephony consists, almost exclusively, of calls made from cell phones, back into the company's various sites PBXs. I have seen such things advertised in Europe, with the extraordinary benefit of (by using a GSM gateway) making all calls appear "inside the GSM network", thus very low cost, or, sometimes, even free ... TIA, Stef