On e-sulat, I beleive Roel used XML-RPC over HTTP. There was a third party SMS gateway provider involved so OpenACS was sort of the middle man between the user and gateway.
On tala-arawan.net, the site only supports MMS messages. I explored the possibility of SMS but I did not want to approach any third party gateway (SMS Aggregators as you call them) and a GSM modem was a bit out of my budget.
