[Nextgrid]

> This is the intended behavior.

You are misunderstanding. I am not talking about sending SMSs from the SIM to the outside world. That would indeed open you up to spoofing and all kinds of abuse.

I am talking about the the other way around. I have a message from the outside world (whether received through a Twilio number but handed to my own application for processing so Twilio's context of the original sending number is lost, or from a different source like Slack or Telegram) and want to send it off to a SIM, using an arbitrary sender number to distinguish between conversations. This fails too, despite there not being any obvious abuse that I can think of.