pin

Reset PIN.

This is basically what you need to do, if you've lost this.  If you use a password-manager, it's a great idea to store this along with your T-Mobile credentials, assuming you're talking about your T-Mobile account PIN here.

If you're talking about your (i)Phone PIN, you can bail yourself out if you've previously associated it with a computer, and you have access to said computer.  If not, you have to simply reset, and (hopefully) use a backup to restore.

This is a decent tutorial on those options: Forgot iPhone or iPad passcode? 6 ways to reset it safely (idownloadblog.com)