The regulation doesn't specify how or whether you contact the customer. You can call instead of write, and if it's more effective because of the personal touch, so much the better.
If you use email for this purpose, be sure that you've got good restrictions on what personal information you include in the message. I'd avoid account numbers if possible, or truncate them.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8