I've sent weird SMS messages through my (rooted) phone before. Silent SMS messages (useful for police trying to activate your phone for triangulation) still returned delivery reports a year or two ago, allowing someone to find out if someone else's phone is on without the target ever noticing.

I think Android removed the ability to send more annoying messages through its normal APIs (like flash SMS messages) but if you have access to any modem (either a smartphone or a full ESP32 dev board costing <50 bucks on AliExpress for 4G LTE), you can bypass any such restriction.

Carrier firewalls will probably filter these messages out but I wouldn't rely on it.

Back in the day you could send logos, ringtones, and bunch of other random crap thru carrier APIs, provided you signed a blood contract with them lol

Those APIs are probably still online, and still require to nail your ass to the contract, but technically I would assume it's possible.

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.

Looks like the website offers an APK.

But not all SMS are text messages for the user.