I'm a commission based employee and want to write off 50% of my phone for business use. does TurboTax calculate that automatically or do I have to do that?

You will enter your cell phone in the telecommunications section. This section does NOT do the calculation for you. You will enter 50% of your phone expenses here.

Do not use the "Office in Home" for your cell phone as this area gets prorated based on the area of the house and office.