If we recieve an oral notification of a disputed charges on a credit card, since it's not in writing, it's not technically a billing error subject to 226.13. Thus, I can take all the time I need to investigate. But, if I start to investigate and then get it in writing, I then have the full timeframe to investigate under 226.13 (2 complete billing cycles), right?

Not necessarily.

If you take the dispute over the phone and tell the consumer, or otherwise give the reasonable impression, that you are absolving him/her of the responsibility to write in (most likely because you are beginning your investigation), you must adhere to Reg. Z.

Verbal statements to the consumer such as "We'll contact the merchant for you" or "We'll send you a letter if we need more information" or "I'll use this information you've given me to do a chargeback" or the like qualify as policy overrides UNLESS you advise the customer IN THE SAME CONVERSATION you are still requiring written notice.

In addition, while a fraud notice not submitted in writing may not qualify as a billing error, it may still qualify as an unauthorized use notice [226.12(b)(3)] and you must evaluate the claim accordingly.