You can claim the cost of using your mobile phone for business.
If the phone bill is invoiced personally then only the business proportion is allowable. You will need to do some work each month to separate out the cost of business and private calls. The rental part of the contract is not allowable and cannot be reclaimed; inclusive minutes are considered part of the rental.
As there is no tax payable on the private use of company-owned mobiles, it makes more sense to buy a mobile on a business tariff, and arrange for payments directly out of your company bank account. The cost of the phone and all business and private calls then receive full tax relief.
FAQ: Can I claim the cost of downloads (ringtones, games, software) to my phone?
A. Downloading a contact management application yes; If you're making your iPhone be a Lightsaber then no.