As a sole trader, you can't claim for the line rental if you're using your mobile phone for business and personal use. You can claim the costs of business calls, if these are indicated on an itemised bill. To claim for the costs, you'll need to set up a mobile contract under your business name, and use it solely for business purposes.
As a limited company director, you'll need to set up a contract between your company and service provider in order to claim the full costs as a deductible business expense. If the contract is made under your personal name, you won't be able to claim the costs as an expense.
Refer to our guide on mobile phone expenses for further information.