When I read about the mod, I got the impression users just selected provider for their phone in a drop down list.
But only the demo has included many, and developer indeed says to contact for information on how to add more.
Seems you need providers info - and insert to gateway file..
http://www.boonex.com/unity/extensions/entry/Free_Sms_For_Members

Free SMS usually means sponsor paid SMS (containing sponsor text), unsure about this one though.
I'm looking for reviews :)

(working on module see more for 2 different senders we use, paid and optionally sponsor paid. considering making it available public if there is any interest for it - or maybe this one does the trick:)