Let's take an example of a single transaction that the consumer states in writing is unauthorized (and I'm only talking about the assertion of a claim, not its resolution).
The fact the consumer has stated this to you automatically qualifies for the Fraudulent Use subsection. That's well and good for the consumer, but you (the bank) have the right to hold him liable for some portion of the disputed amount under this section (Reg E and Z differ as to the amount and conditions for this).
Whether it qualifies as a Billing Error requires that the consumer send the written notice within the proper time frame. If it does qualify, the consumer has no liability at the time of assertion because the protections under Billing Error are stronger than the ones under Fraudulent Use.
Verbal claims always qualify under the Fraudulent Use section but don't qualify under Regulation Z or E unless you advise the consumer the he does not have to write in (but I can't imagine that you would allow this). If you should ever receive the written notice, you would then go back to the appropriate Billing Error section to determine if the asserted dispute now qualifies.
Two notes:
1) The consumer's cut off date is the date the notice is sent to you, not the date you receive it.
2) Only Reg E explicitly states that the consumer retains the right to upfront credit under Billing Error protections unless you request confirmation in writing. Reg Z does not actually address the matter at all. However, many chargeback rights don't require documentation and you could conceivably have a policy that allows you take the details verbally from the consumer and then submit them as part of a chargeback. I
f you take a dispute verbally with the 60-day time limit, say for undelivered merchandise, you have in essence absolved the consumer from the documentary requirement even if you don't do anything with the information. You could not come back later and then deny the Billing Error because the consumer didn't write in.
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My opinions do not necessarily reflect those of all the voices in my head.