That seems unlikely when I get a response that says “You have been unsubscribed - Bob Loblaw for Senate” or whatever. I suppose that could be pre programmed.

For which countries is this true?

And if I send a reply to my friend with just the text "stop" - that does it, right? I mean part of normal conversation, not to indicate that they should stop messaging me. Or should I remember that it's the special phrase?

For what it's worth, I've sent a "stop" before and gotten this:

> NETWORK MSG: You replied with the word "stop" which blocks all texts sent from this number. Text back "unstop" or "start" to receive messages again.

I assumed it was from my carrier (T-Mobile in the US), but now I'm wondering, as I have gotten different replies from other numbers. Maybe it came from the sender's provider? Or is just misleading.

That's not true at least in the USA on my carrier. Some spammers just ignore the STOP and continue messaging anyways.

Every spam message I get comes from a different number