As I get asked this every few days decided to post this thread, Maybe Admin can sticky it
Now the cheapest way of getting a factory unlock is via your network provider (eg Vodafone, O2, Orange)
Some will do for free and some will charge (i will update this every so often with network prices and links etc)
Networks will quote a lead time of upto 28 days but usually its 2-3 days, you connect to itunes and thats how the unlock registers to the phone once all apples servers are updated.
Links to request unlock
Vodafone, free if out of contract £19.99 if pay and go or in contract they can refuse if brand new contract
O2 again can be free if out contract, £15 if pay and go or in contract
Orange/t-mobile/EE most fussy of the networks to get unlock since EE merger £20.42 must be customer for at least six months
Three, need to be customer at least 30 days £15.32
Tesco free after 12months , wont prior to that
Now if you handset has been swapped out by apple then the network wont have this currently within there database but they can request it
you need to supply the apple paperwork to the network and they should organise this for you, it may have to be escalated but it can be done !
If you have bought a locked handset of someone or ebay etc and want it to be unlocked you can use the above links once the sim has been latched to a valid account as above, place sim in the phone and use if for min 48 hours to latch that imei to your account then complete the above process for your network, now if this has been an apple swap out again this could cause you some headaches.
Finally the other option is for 3RD PARTY factory unlock, you will see loads of different prices and sites for this but they all go via a dozen or so direct sources that have access to do this, rough costs are as follows and lead time.
3rd Party factory unlock prices always changing due to currency and availability 3-9 days is current leadtime
Three £ 50
Id be very dubious of prices lower than this as my source is direct, the supplier could just be doing it via the network and charging you for the pleasure.
Finally iphones purchased from other country's can be unlocked costing anything from £5 - £105
Hopefully this clears up and confusion and just shout if not sure