You have a number of options where employees use their personal phones to make business calls, each with different Tax and National Insurance implications:
If you reimburse the cost of the mobile telephone, service charges or the cost of private calls, the amounts must be assessed through the payroll for Class 1 NIC and tax. This treatment also applies where you provide the employee with a voucher for use in relation to the mobile telephone or private calls made on it.
If you just reimburse the costs incurred by an employee in making business calls only on his or her own mobile telephone, you can ask for a dispensation from the HMRC. If you do not have a dispensation you must return the full cost you have incurred on reimbursing the calls on form P11D at section N. No Class 1 or Class 1A NIC is payable. The employee can then apply to the HMRC to advise that the payment is for expenditure incurred in the course of their employment and claim a refund for tax paid on this.
An alternative option is to provide these employees with a mobile telephone for business and enter into a service agreement with the telephone company. Then there would be no liability for NIC or tax. This applies to the provision of the telephone, the service and all private and business calls.