Years ago my accountant told me not to take 100%, red flag. We deduct 90% of our cell phone cost. One of our cell numbers is actually listed at the end of our leases. 99.9% of the calls are business related. However, there is always the occasional personal call, I know I can prove more than 90% at anytime. I also take a % of my internet connection expense and all of my computer security costs, too.