You can’t deduct the monthly fee for the first phone line into your home, whether it’s a cell phone or a landline. You can deduct a second line (or more) if you use it for your business. Use your time-space % to determine the business portion of a second or third line. You can decide which is the first line into your home.
If your phone, Internet and cable TV bill is bundled together, determine your business deduction following this formula: divide the total monthly bill by three. Deduct the time-space % of your Internet and cable TV portion (assuming you use these for your business. Divide the last third by the number of phone lines into your home. Don’t count one of the lines. Deduct the time-space % of other phone lines used for your business, unless one or more lines is used exclusively for your business. If so, deduct 100% of an exclusively business line.
You can deduct other costs associated with a first phone line: the cost of the phone, caller id, long distance calls, etc.