How much does it cost to call an 02 number from mobile?

An 02 number is a normal UK geographic number. The cost of calling it from a mobile will vary from provider to provider. The rate will usually be between 3p and 55p per minute for calls made from within the UK.
However, don’t forget to check your call package with your provider. 02 calls may well be included, in which case you will only pay if you have exceeded your minutes.
Calls made from abroad may be more expensive, so if you are planning to go abroad, check with your provider that you are on a suitable plan. You may be able to get an add-on that will reduce the cost of calls to 02 numbers.
If you think you have been overcharged, then contact your provider and if they are unhelpful contact Ofcom.