The city of Minneapolis has a new 411 text service that lets you send tips on miscreants in total anonymity. Easy and straightforward.
The news release also notes that there's an app for your smartphone that you can use to send tips. The app's page on iTunes has screenshots that show how you report a carjacking on a phone without dialing. Finally! This will work well:
Tipster: I saw a cat jacking on Lindale
Response: please call 311 for Animal Control thank you for using MPD Tip
Tipster: LOL no srsly car jacking the guy had a pistil
Response: okay did you see which way they were going
Tipster: No I was looking down at my screen typing
To be fair, the first thing you see is, "In case of emergency, call 911," and carjacking would seem to fit that category.