Our apologies, our previous answer was wrong. We hadn't previously been asked about the specific situation of an employer paying the employee's mobile phone bill diirectly where the contract is in the employee's name.
You need to set up a specific payment code for paying the employee's phone bill with the NI button ticked but not the tax button. You add this amount as a payment to the employee. You then also set up a specific deduction code to reflect the fact that the employee isn't receiving the money from you directly, with neither the Tax nor the NI buttons ticked, and you make an equal and opposite deduction on that code. If the contract is a fixed monthly amount then you can set up both the payment amount and the deduction amount once on the employee's "Pay" tab.