O2 PAYG Online charge 10p a min to phone 0844 no,s on weekday evenings.So if you were phoning a Voda Mobile you could use  the 08444 804 700 number mentioned on this site ,and save yourself a fair few pence.O2 PAYG normally charge at least 40p to phone other networks,

NOTE. THIS NUMBER ONLY WORKS IF YOU ARE CALLING VODA NUMBERS OR NUMBERS PORTED FROM VODA ON WEEKDAY EVENINGS.

Please accept my apologies if this idea has been mentioned before.