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What type of unlock code are you looking for. A carrier / subsidy unlock code so you can use that phone on a different network? ( If so, what network are you trying to move the phone to)
If your service is through AT&T still, you might want to consider calling them and asking them for the subsidy unlock, or network unlock code since you will be traveling soon. If the phones contract is paid off, they shouldnt have any issues giving you the code.
If they wont give you the code ( its worth asking first ) You can order an IMEI unlock code from websites similar to this.
The 1-5 day service costs 9.99.. Not a bad price for an unlock code as they average around 30$