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>If you have knowledge of a withdrawal and even a rough ball park of that amount, then you can probably determine it was a phone purchase.

I mean, this is only true if you pull out money once just for this one purchase. If you pull out money regularly and make most of your purchases with that cash, it would be incredibly difficult. If I pull out $500-$1000 per week in cash, with occasional $1,500-$2,000 weeks, there would be very little ability to know if I was buying a phone on a $2,000 week or a car repair. If a $1,500 week was a regular week with a fancy dinner, or a low expense week with a phone purchase.

It would be like trying to tell what you bought with the credit card you use for all your expenses by only looking at the size of the payment each month from your bank account.