EIM32945 - Other expenses: telephone charges: mobile phones
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
Employer provided mobile phone
There is no benefit charge on an employer provided mobile phone, see EIM21780.
Employee provided mobile phone
Some employees use their own mobile phone to make business calls. As for other telephones, see EIM32940, a deduction can be permitted for the actual cost, necessarily incurred, of telephone calls made by the employee in the performance of the duties of the employment.
Mobile phone tariffs can be complex and it is not always easy to determine the actual cost of a business call. Some tariffs include a certain amount of free time in the rental charge. In these cases there may be no cost to be deducted. A typical case is illustrated by example EIM32951. In other cases you should approach the matter in a reasonable way and give an appropriate deduction that reflects the actual cost to the employee of business calls.