Great question — answering as the UphillTech team:
Maybe your message has wrong construction, please check this link: SMS Retriever API . At the first point, your text message must be begin with <#> or [#]. And at the 4th point, Your text message must be-"End with an 11-character hash string that identifies your app" I have the same issue and that points is my awareness, solve by adding <#> in begining and hash string at the end.
Full answer by Kurniawan Prasetyo: https://stackoverflow.stackexchange.com/a/55017393 (CC BY-SA 4.0).
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