u dont need PUK code ever except when ur phone gets locked........the phone gets locked if u enter the Pin no. wrong 3 times. then it asks for the PUK code which if u put wrong 10 times, ur sim will get blocked permanantly, then u will have to buy a new sim, u will get the same phone number but u will lose all the data on the sim.....
BTW PUK code depends on ur sim, even if u get ur sim replaced, just casually , the PUK code will change. each sim has a puk code, which itself never changes. so if ur phone gets locked again u can punch in the same puk no. once again to unlock it.