To be deductible, a business expense must be both ordinary and necessary. An ordinary expense is one that is common and accepted in your trade or business. A necessary expense is one that is helpful and appropriate for your trade or business. If it is an accessory to your cell phone and you feel it meets these criteria, you could include it as part of cell phone expense.
If you use it for both business and personal, you must allocate the expense using a percentage. You can determine this percentage from your monthly bill, call log or something similar.