Allowable expenses

Hi, I rent out 2 properties which I manage myself. I was thinking of buying a tablet computer to use for the advertising of the properties and general management like excel documents. Obviously from time to time it would be used for non business purposes but the sole reason for buying it is business. Is the upfront cost or the monthly charge for data a reasonable expense?
Also how do you go about claiming expense for phone calls? Is a set price per call similar to car allowance of 45p/mile?
Thanks in advance