There are two general treatments to a business/personal cell phone. One argument is that you can deduct the percent of business use IF you can determine it from your records (call lists, etc.) If you can't determine percent of business use, it's not a deductible expense.
The other argument is that, even if you can determine percent of business use, you can't deduct a percent of a flat rate plan since you would pay the same for personal only service, therefore the business doesn't cost you anything extra. I believe there are some tax court cases siding with this argument although I do not have links.
But even if you want to use the first argument and deduct the percent of business use, you have to be able to prove what that is from your phone records.
