怎樣unlock iPhone 4

我有一個iPhone 4，今年合同到期後，從ATT得到了unlock 的許可。按照ATT 的指示，我隻需要走3個步驟就可解鎖：即backup，reset 和restore。但我做了後拿到另一家公司去檢查，卻說是沒有解鎖。請教：怎樣才能解鎖？怎樣才能知道解鎖是否成功？謝謝。
附：ATT 的解鎖email：

Request number: xxxxxxx
Thank you for contacting AT&T Customer Care about unlocking your iPhone.

We have reviewed your request and confirmed that you are eligible to have the requested iPhone unlocked.

Please allow 24 hours upon receipt of this notification to complete the unlock.

To complete the unlock, simply:

1. Open iTunes on your Mac or PC and verify that you have Internet connectivity.
2. Ensure a SIM card is inserted into your iPhone.
3. Connect your iPhone using the dock connector to USB cable that came with your iPhone.
4. Backup and restore your iPhone using iTunes. For information on backup and restore, please visit http://support.apple.com/kb/HT1414.
5. After restoring, your iPhone will be unlocked.

Additional information on unlocking can be found at http://support.apple.com/kb/TS3198

You may also check the status of your unlock request by clicking the link for AT&T's Device Unlock Status Portal.

For questions regarding AT&T's wireless service and iPhone, please visit the Phone/Device Learning Center.

We thank you for your continued business.

Sincerely,
AT&T Customer Care Team