Lightweight messaging clients?
 

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.

Can you tell me what was the URL?

At this moment I can't but I should have been clear that the callback was from CallbackWold (CBW), a now defunct callback service that was popular with the members of this forum. They had a callback trigger via web page. I realized that this page just created a URL and the URL actually triggered the callback. By saving the URLs of commonly called number combinations on my phone I could trigger callbacks with virtually no roaming data usage.

Some of the sms substitute services use only 1 to 2 kilobytes of data per message

I know that's not as low as perhaps 150 bytes of a URL, but it might be worth a look