When the opt-in rule was in proposed form, your scenario was part of the discussion in some of the comments submitted. The Fed at the time said that they were not going to provide support for that type of opt-in, and there's been no change since then.
There is no requirement that the opt-in be in writing (although both the disclosure and the confirmation must be unless E-SIGN applies). If you have a process that allows for phoned-in opt-ins, you need to have good documentation for the calls.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8