No, you can make any upi id, like emails. Apps, being lazy, start giving phonenumber@app id to its new users. I always have used custom@app upi ID.

I’ll avoid repeating my previous comment [1], which has details on this. Unlike the mistaken belief of several people on this post, UPI does share the phone number even if a person uses an alphanumeric UPI ID instead of the initial or common phonenumber@provider address.

[1]: https://news.ycombinator.com/item?id=37371646

Not the protocol itself. Your ID is an arbitrary alphanumeric string that you register with a provider. But most people just use their phone number.