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It sounds to me like you may need to manually rescan the Network Operators setting.
Select a different network, then scan again and select Vodafone. This should lock you back onto the Vodafone Network.
Wireless & Networks > Network Operators > scan...
The only other thing I can suggest is to Factory reset the Phone.
When you do this it removes the Network association.
During setup you will see a pop up window which asks you to choose your Contract type;
Vodafone Contract = Monthly contract
Vodafone PrePay = Pay as you go.
This then automatically sets the phone up with the correct Network settings.
The only other thing I can think of is some sort of account bar has been activated on your account in error?
If your phone is failing to recognise 2 SIMs, then it could be a hardware fault. However, there's one simple thing you can try: simply clean the SIM contacts. Just wipe them firmly with a soft cloth. My S2 wouldn't pick up a network until I did this.
The Unlock code you have requested, is to unlock a Phone for use with other Networks.
All Vodafone Pay Monthly Contract Phones are unlocked by default. So there is no point in getting this code.
Have you visited a Vodafone Store? It might be worth letting them take a look. They can provision a new SIM in Store, which may help?
It would at least point to hardware then...?
I've Googled this a bit and there are various solutions, usually unconvincing or requiring rooting. It does rather sound as though the code from Samsung is what you will need, but why this happened in the first place is a mystery and it doesn't seem to be particularly common. I suspect something has corrupted the operating system.
If the code doesn't solve the issue, then it might be best to go to a Samsung repair centre in person and show them what's happening. I'd suspect eiterh a hardware fault or that the phone needs completely wiping and re-flashing.
Once an NUC request has been confirmed as submitted to a manufacturer then it’s just a case of waiting until they advise us what it is and then we can let you know.
I must admit to have not heard about this specific issue. If the phone has been locked to T-Mobile, then I would presume they’d need to supply a NUC as a Vodafone one wouldn’t work.
I too had Enter Network Lock Control Key issue recently, after updating my Galaxy S GT-I9000. I got it solved just yesterday thanks to the application called Galaxy S unlock which I got from the Play Store. The solution was amazingly simple and fast. But your phone must be rooted for it to work. I am confident that the owner of the application can help you solve your problem.