I’d suggest using TextAnywhere:
Text Anywhere
They provide API’s (C, Java, .Net, PHP etc) for several languages and provide a multitude of rates and packages depending on the volume of texts sent.
They’re also very flexible with regards to payment – they do pre-pay, PAYG and fixed monthly limits.
Pricing
We used them for a large-sized application to send alert notification messages and they were very reliable. They also give 15% discount for registered charities and academic institutions.