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Obviously, that makes it difficult to create a national service. That same lack of truly national coverage would likely be a barrier to consumer take-up, which in turn reduces the willingness of the regional operators to consider any sort of scheme.
I think your best bet may be a series of regional agreements. Look at the geographic spread of your US vsitors (cross matched with who buys if possibe) and try to discover which cities/states are bringing you business. Find out who the local mobile players are, and whether they have any micropayment systems in place.
The ODP is usually a good starting point [search.dmoz.org].
Maybe some of the US members know more about the specifics of who operates where, and which services are offered could offer more insight
The larger companies (ATT, Sprint, etc) do offer *almost* nationwide phone service, but data/internet phone service is much more limited, geographically. Here in Alaska I can't get anything more than basic phone service, and not even from any of the big national companies.
All in all, we still use our cellphones mostly for talking over here...