If you don't have a dedicated business only phone and Internet, then there are two schools of thought on deducting the expenses.
One argument is that you can deduct the percentage of business use from the plan cost, but only if you have a reasonable way of determining the percentage of business use, like a diary, log, or other record.
The other argument is that if you have a flat rate monthly bill, you can't deduct anything for business use, because your bill would be the same whether it was 1% business or 90% business, therefore the business does not increase your cost.
I personally believe that the second argument is more correct, although I don't have an IRS reference handy. Even with the first argument, you can only deduct the percentage of your bill if you have a reasonable way of showing went at the percentage of business use was.
Of course, if you establish a separate phone and Internet connection for business only use, then you could fully deducted.
