Here's the direct answer: the best prank you can pull with nothing but someone's phone number is signing them up for daily cat facts, because it is anonymous, legal, harmless, and the target can end it with a single word. Everything else in this category is either a worse joke or a worse idea. Below is the honest version of the list: what works, what to skip, and why the line sits where it does.
Why is cat facts the best phone-number prank?
Because it checks every box at once. You need only a number. You never touch their phone or install anything. The messages come from the Cat Facts number, not yours, so your name stays out of it. And the content is real, wholesome, and slightly absurd, which is exactly the flavor a good prank needs.
The recipient gets a text explaining they are now subscribed to cat facts. Then the facts start arriving: one a day on the $2.99/month plan, or up to 15 a day if you want to make a point. They can reply STOP at any time and the texts end immediately. Every message says so.
That is the whole prank. It has been making people laugh since a Reddit thread in 2012, and it has aged well for one reason: nobody gets hurt.
What should you NOT do with someone's phone number?
Here is the short list. Every item on it has ended a friendship, a job, or a clean record for somebody.
Fake emergencies. "Your mom is in the hospital" is not a prank. It is a cruel text, and depending on where you live, it can be a crime.
Spoofing or impersonating. Sending texts that appear to come from their boss, their bank, a delivery company, or a government agency is fraud territory. Caller ID spoofing laws exist for exactly this reason.
Anything the law calls harassment. Repeated unwanted messages with no way to opt out, threats, sexual content, or anything designed to frighten. If the target cannot make it stop, it is not a joke.
Anything involving their money or their job. Signing them up for paid services they cannot cancel, texting their employer, poking at their accounts. None of this is funny, and all of it is traceable.
Posting their number publicly. Classified ads, "call me" posts, dating profiles. This one gets people doxxed and gets you sued.
Quick test: if the prank only works because the target is scared, confused, or out of pocket, skip it.
What is the safe category?
Wholesome, opt-out-able recurring texts. The target should be mildly baffled, then amused, then curious about who did it. They should never be worried.
Cat facts is the most famous example of this category. It's one of the few pranks where the recipient sometimes ends up wanting to keep the subscription.
How does cat facts stay on the right side of the line?
Four things, and each one matters.
Consent to end. Replying STOP stops it. Instantly, permanently, no questions asked. The target holds the exit the entire time.
No impersonation. The texts say they are cat facts. They come from the Cat Facts number. Nobody is pretending to be a bank, a boss, or a grieving relative.
Real content. Every message is an actual fact about cats. The target learns something, which is more than most texts deliver.
No cost to them. They pay nothing beyond their standard message rates. You cover the plan, starting at $2.99/month.
We covered the legal details in Is the Cat Facts prank legal?, and the anonymity side in anonymous prank texting. The short version of both: a joke with an off switch is a joke, and a joke without one is a problem.
Will they figure out it was you?
Not from the texts. We never reveal who paid. Whether you confess or let them stew is entirely up to you. Most people crack within a week, mostly because they want credit.
How do you actually do it?
Go to the signup form, enter their number, choose how many facts a day, and confirm. The first text usually lands in seconds. If you want to try it before you commit, there's a free three-text trial on the homepage.
Pick a friend who can take a joke, and pick a number of facts. The prank that gets you in the least trouble is also the funniest one. That is not a coincidence.