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You need to call your network provider and get this enabled on your account as the first step (your network provider must support this before it'll work on your phone).
Once enabled, you make a call as normal - with the caller on the phone, goto the menu and select the option to make a new call. With the new call made, press the left soft key (the left one under the screen) and an option should appear on the list which says "enable conference" or something similar.
I've been through pretty much all of O2's website and no where on their website can I find a list of features which their network supports !
I can find the list of supported services on Orange's website and they specifically state that conference calling is supported.
I've had a look at the Nokia N73 as an example on O2's website. Nokia's website specifically mentions that this phone supports conference calling however O2's website makes *NO* mention of this at all ... so given this information I'd say that O2 doesn't support it - but again - check with your network provider for the definitive word !