Some time back I figured out a way to trigger a callback service by simply "touching" a URL from my mobile phone. The point was that the data usage for doing this was so low that I never saw any data roaming charge on my account.
This has made me wonder if there might be a very low-overhead messaging protocol, perhaps using
UDP packets that could substitute for SMS and be able to more-or-less fly below the billing radar, or be so small that the billing would be insignificant.