Yes you can claim for a mobile but you would be better off claiming for a %.
Again, the only reason being that if your found to be claiming for a mobile that you also use for personal usage then the tax man will investigate further into your records and may find all sorts of irregularities.
How will he know, well you explain why you made a call at 1.30am in the moring for a taxi having been out on the beer, or why your txting mates at silly o'clock in the morning, all of these show up on your itemised bill which the tax man is legally entitled to ask to see??
Again, a small gain on your part may cost you dearly in the long run. Its like being stopped by the police and you smuggly prove you haven't been drinking, suddenly they find something else to do ya for!!!