I don't have the real answer, but I remember back when Google Desktop was a thing, I had a widget that would allow me to send a text to anyone and it had a fiel

I don't have the real answer, but I remember back when Google Desktop was a thing, I had a widget that would allow me to send a text to anyone and it had a field for the "from" number. So my guess is that the protocol they use asks the caller for the "from" number.