If the Mobile contract you can claim the 100% cost of the bill and the VAT as an expense. Even though there could be personal use of the mobile, the company or employee does not have to pay any Benefit in Kind taxes (BIK).
However if the Telephone / Mobile contract is in the employee’s name, it gets little complex. So, if the employee use their personal mobile phone to make a business call only the actual call cost necessarily incurred for the business can be claimed as an expense and not the line rental. Please see the example provided by HMRC.
A construction engineer often works out of the office on construction sites. She uses a mobile phone so that she can keep in touch with the office. The phone is mainly used for business calls.
The tariff for the mobile phone includes up to 10 minutes of free calls each month. In one month she pays £22, which is the rental charge only, because her total calls, all of which were business calls, amounted to 8 minutes. No deduction can be permitted because no expense has been incurred in making the business calls.
The following month she pays £28, which is £22 for rental and £6 for calls. Calls that are charged are paid for at a rate of 20p per minute. In the month she made calls totalling 40 minutes, of which 30 minutes were for business and 10 minutes were private. A deduction should be permitted for the cost of business calls, see EIM32945. The amount that can be deducted is £4.50, which is 75% of the call charges, because 75% of the total call time was made up of business calls.
We recommend to have the mobile contract in the company’s name to claim 100% of the mobile cost and to enjoy the tax free benefit.
In the meantime, if the employee have two mobile contracts and one mobile is used 100% for the business purpose, then this cost can be claimed proven it’s used only for business use.
As far as Broadband costs incurred by the employee at home, if the employee is already paying for a broadband contract, the employer cannot reimburse the cost as there will be no additional cost to the employee for using it for business. If the employer decides to reimburse the broadband cost, this will be subject to PAYE and NIC.
In the meantime, if the employee does not already pay for a broadband internet connection at home, and needs one in order to work from home then this cost can be reimbursed by the employer without any PAYE or NIC implications.
Please refer HMRC’s guidance here