Neither my wife nor I would want a fob if the phone connection were reliable 99% of the time. But for us it is reliable maybe 50% of the time.
Tesla spent money to design, test, and configure this means of access to our Model 3s, and we know that the costs incurred are baked into the selling price of the vehicles. Just how much we do not know. But I would surmise that if the only access were the little credit-card sized backup plan, that the price of the vehicle would be a few bucks less, or we would have something else in its stead that might actually work.
In essence, Tesla foisted upon us the phone access, and this access method does not work in the way that it was intended. Nor does Tesla know why, or if they do, they are not doing anything about it. Service and the telephone folks are helpless to resolve the issue.
We are not angry or upset. Merely frustrated and disappointed. At least we can drive the car, even if we have to fumble around with the little card, keep it handy, and remember to use it to lock the car upon exit.