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The iPhone is not unlocked via inputting an unlock code like other phone makes are, but instead is unlocked remotely via Apple and their Database.
The Vodafone Network typically lets a person know it's unlocking has been requested to Apple after accepting your online unlock request. (Please Check Spam Inboxes too) The iPhone isn't actually unlocked at this stage.
Apple then process the Networks request and occasionally this can take approximately up to 30 days sometimes to complete an unlock request made by a Network.
Please Periodically insert an active Alternative Networks Sim Card in the iPhone and hooking up to iTunes is only typically needed if its an iPhone 4 or older.
Some people were experiencing issues with the unlocking page not too long ago but that seemed to have resolved itself. Can you try another browser or clear the cache and cookies of your current browser.
Another option is to ask customer services by phone or Live Chat to fill out the form on your behalf.
Another known option is to pick up a Payg SIM card from Vodafone and use it in the iPhone for 30 days making calls etc and once that timeframe is up then use those Payg details to ask for it to be unlocked.
Just please take into account the iPhone isn’t unlocked by a physical code for you to input into the iPhone and the other information I mentioned earlier.