As a result, people with mobile calls included ain their landline package are being charged for calls to this number.
Also, if the number calls my (Android) mobile, or indeed anyone elses Android mobile the number shows up as "Spam". Now I know that's Google putting that interpretation on it but why would it for a standard, run of the mill mobile number if there wasn;t something out of the ordinary about it?
07305 are standard mobile numbers and have been allocated to Virgin Mobile since November 16.
Could be a recent issued batch of numbers being used, this can result in being seen as a premium numbers to other operators that are slow updating their records to include the new numbers as standard numbers on their system.