I think the guarantee is the tough part; you have to own the hardware and have at least some control over the infrastructure.
Anyway, I find Georgie's comment fascinating. Sounds like there's a way to guarantee SMS delivery, though. From Wikipedia on SMS/SS7[1]:
> Therefore, it is possible to avoid delays and message losses, offering full delivery guarantees of messages and optimized routing. This model is particularly efficient when used in mission-critical messaging and SMS used in corporate communications.
Anyway, I find Georgie's comment fascinating. Sounds like there's a way to guarantee SMS delivery, though. From Wikipedia on SMS/SS7[1]:
> Therefore, it is possible to avoid delays and message losses, offering full delivery guarantees of messages and optimized routing. This model is particularly efficient when used in mission-critical messaging and SMS used in corporate communications.
[1]http://en.wikipedia.org/wiki/SMS#SMS_gateway_providers