I'm not sure about special requirements, but I know Servage.net provides you with a SMS platform and a free API to develop your scripts, even though you need to buy SMS credits besides your hosting payment that only includes 5 for free.
Your question is very general which requires a broad answer. You should give much more info to help me answer in a much better way. For example; what is your target mobile users, which operators are you targeting to, how big is your target user etc.