Pub 535

Utilities. Your business expenses for heat, lights, power, and telephone are
   deductible. However, any part due to personal use is not deductible. 

   Telephone. If you have an office in your home, even though you are in business,
   you cannot deduct the cost of basic local telephone service (including any taxes) for
   the first telephone line you have in your home. 

   Cellular telephone. Generally, any cellular telephone or similar
   telecommunications equipment is listed property. If listed property is not used more
   than 50% for qualified business use during any tax year, special rules apply to the
   section 179 deduction and the depreciation deduction. See chapter 4 of Publication
   946.