I saw an article on c-sharp corner some time ago that used a service called way2sms. It is for India only so was of no use to me.
The author created a web app that interfaced to a page on the way2sms site.
http://www.c-sharpcorner.com/UploadFile/diya0076/827/
They might have other interface options available, but I didn't investigate further.
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