I made a test page: https://ericwuapps.com/test-sms
Use it on a smart phone, click to be ready to send a text message. it fill out the recipients and body automatically. everything looks good, I can hit the send button to send it out, but none of the recipients is actually receiving it.
I’d check if the phone number is in the right format. Can’t be sure off the top of my head but I don’t think you should include the +. Just country code followed by number. No spaces, no punctuation.
Would you consider the users sending the SMS through your system, as in you sending it on their behalf with the “from” being their phone number which you capture earlier in the process? That would be platform independent, but of course, it would cost money and the users would not see the sent message on their own phones.