Will a phone in a Tesla cause it to lock?

How to unlock your car with the Tesla Phone Key feature is described in this article.

The Tesla Model 3 does not automatically lock when you walk away, but the key fob and phone key app are intended as backups.

To set it up, you must first launch the Tesla mobile app and then tap the phone key.  

Then, all it takes is a quick tap of your smartphone to unlock your automobile.  

In addition, you may unlock your Model 3 with an extra key card, giving you another fallback option in case you lose your phone or key fob.

The Tesla's ability to lock when your phone is inside may also be bypassed using the key app phone.

You won't have to worry about leaving your car unlocked if you use the key fob that was developed for the Model 3 and can detect proximity.