You both will enter your own expenses and allocate it based on business use. For example, if the total phone bill is $80 then you can allocate $40 to you and then $40 for your husband if that is how much each of you paid.
Then, you will further allocate your $40 phone bill based on the business use portion. For example, if your phone was used 25% of the time for business, then you can only take a 25% deduction. The rest 75% is not deductible since it was for personal use.