To claim full relief on your mobile phone or telephone you will need to ensure that a contract is set up between your company and the service provider, this way you can gain full tax relief on the cost of the phone and its use - any personal calls on the mobile are treated as a tax free benefit in kind, so are a consequence of an allowable business expense and will encounter no tax - or need reporting on your P11D.
If you take out a phone contract in the name of your business you will also receive Corporation Tax relief on the entire cost of your phone bill, so if the entire balance of a mobile and its contract is £1,000, this would be recorded as an expense, knocking £200 off your Corporation Tax bill. The personal use included in the bill would be classed as a tax free benefit in kind, so you will not be taxed on it.
The one caveat with signing a phone contract in the name of your company is that youll probably have to use one of your phone providers business tariffs, which may prove slightly more expensive but any increase in cost will probably be wiped out by tax savings.
If the contract is under your personal name, you are able to apportion the business costs, providing that you can demonstrate they were business made.
No percentage of tariff can be claimed on basis of paying for minutes because these minutes are "free" and therefore have no cost.