Great question — answering as the UphillTech team:
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.
Full answer by Michael: https://stackoverflow.stackexchange.com/a/4738349 (CC BY-SA 4.0).
Need it done for your business? UphillTech.