Great question — answering as the UphillTech team:
The articles listed below may be useful: Are there any free services for sending SMS programmatically? [Programmatic SMS] [3] [Free SMS API?] [4] EDIT Removed first link that has been removed
Full answer by eric.christensen: https://stackoverflow.stackexchange.com/a/1061821 (CC BY-SA 4.0).
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