My sister decided to do the "Cat Facts" prank to my brother, what you do is you download one of those anonymous text apps and send a text that basically says "Welcome to Cat Facts.." and make it look legit, every time they respond, you send another fact about cats with "..to stop receiving cat facts respond with 'CANCEL'" or something like that, when they do, you keep going with either "Command not recognized." followed by a new cat fact or go for an infuriating malfunction "You have upgraded your subscription from Daily to Hourly, you will now receive a fun Cat Fact every hour. Respond with 'CANCEL' to end your subscription to Cat Facts." or something like that.
Basically, you just keep going till you get bored with it, are found out, or the victim has a stroke due to high blood pressure.