1 October 2014 and After:
Communication services are covered in the IE portion of per diem rate and are not reimbursable. Gov’t owned/leased services should be used for official communications. Expenses, such as cell phone or prepaid minutes use, necessary for official TDY travel are mission-related expenses and not payable under JTR authority
See The Joint Regulations (JTR), APP A1 'Per Diem Allowance' definition.