Teslas come with key cards, which are the size of a credit card. My Android phone is my primary key, but I always have a card in my wallet. Fobs and phones can be lost or malfunction. The card is a backup.

Fact check. Tesla uses Bluetooth not cellular to connect to the car. Here is the correct reference.
From the owners' manual
Model X supports three types of keys:
Phone key - You can set up your personal phone as a "phone key" that communicates with Model X using Bluetooth. A phone key supports automatic locking and unlocking.
Key card - Tesla provides a key card that communicates with Model X using short range radio-frequency identification (RFID) signals. The key card is used to "authenticate" phone keys to work with Model X and to add or remove other key cards, phone keys, or key fobs. Unlike the phone key and key fob, the key card does not support automatic locking and unlocking. In situations where your phone key has a dead battery, or is lost or stolen, use your key card to unlock, drive, and lock Model X.
Key fob - The key fob (if equipped) allows you to press buttons to open the front and rear trunks, and unlock, lock, and drive Model X. The key fob also supports automatic locking and unlocking, if available in your region (see Passive Locking and Unlocking) and can be used as a backup to your phone key.