I think you are allowed to take a percentage and claim it as a deduction. If you use your phone for business 30% of the time, claim 30% of the costs of the phone and monthly fees. What I don't understand is what is considered "business use." While being available on your cell phone while on reserve, is that considered business use of your phone? I think it is and would argue that my cell phone is at least 90% business use, but since my airtime is used more with my friends and family, many would say that this is not the case. Anyone with thoughts on this?