How much is what you need to know to explain to the IRS. If you keep track of your phone calls, the minutes used, the purpose of the call (business or personal, and if business, specifically how the call related to the business you were attending to) Then you have to calculate about how much you use your personal phone for business...I'm assuming you're using a personal cell phone, because if you have a business cell seperate from your personal, then the business cell should be mostly deductible...by mostly, I mean 90-100 percent. With your personal cell, you need to calculate how much is personal, percentage base, and how , much is business, percentage base, and you need to be able to prove that the business use IS business use if you are audited. Sounds like a headache, but it's worth it. Even if business use turns out to be only 10%, it's a good habit to develop. This is not financial or legal advice. This is free information that I have educated myself in. I strongly recommend searching the tax laws of your city, county, state, as well as the federal laws that apply to your business(es) at your local library or bookstore. I also recommend that you speak to a certified tax preparer, and accountant or attorney as needed. You are responsible for your own actions.