If you require written confirmation as a condition of providing provisional credit, I agree you shouldn't provisionally credit the account until you receive the confirmation (and you don't ever have to provide it if the confirmation doesn't reach you until after the 10th business day following the oral claim).
One caveat, though -- if the claim involves both Regulation E and a debit card transaction subject to Visa's zero liability requirements, you have to provide provisional credit by the fifth business day, and I believe you can't condition it upon receipt of the written confirmation (Brian will confirm that or set me straight, I am sure).
John S. Burnett
Fighting for Compliance since 1976
Bankers' Threads User #8