Does anyone have a good list of international airtime top up providers?

As more and more people become dependant on their mobile phones, mobile airtime is becoming more than a luxury item but rather a necessity of daily living.  This is especially true the mobile penetration rate continues to climb in developing countries in Africa, China and India.   This is phenomenon is opening up a niche market for airtime top up providers who can provide migrant workers the ability to top up airtime for their loved ones living cross borders.

I was trying to search for information on which companies currently provide international airtime top ups. It was difficult to find which airtime top up providers actually offer global airtime top up.  This is probably because it is such a new business model and there still many kinks to be worked out.   For instance, should airtime value be viewed as equivalent to monetary value?  In some developing countries, airtime can be used to purchase goods and services.  Thus, should regulators track and regulate the transfer of airtime value?  How will the currency of airtime and the exchange rates be calculated and maintained in the global economy? After some time searching, I was only able to find a few providers.  

Most of my search results came back with TransferTo announcements.  This was probably because TransferTo recently announced partnership engagements with Hyperwallet (March 2010 - Canada), Wataniya Telecom (February 2010 - Kuwait) and STC (November 2009 - Saudi Arabia).  

It was interesting to find that Mi-Pay also entered into the international airtime transfer space early this year.  Other international airtime top up providers that I found included Atos Worldline and Belgacom.

Based on my search results, I would say that the market is primed for more international airtime top up providers.