Comment by lesspassiveobse

They do accept/forward messages without checking if they are special carrier-only ones. ~10 years ago I had some fun sending a "voicemail received" message to people, which caused android phones to display a notification which couldn't ever be dismissed. IIRC it only worked on HTC phones and required root/xposed. SS7 in general is built with total trust in connected systems. Edit: I think it was called "HushSMS"