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I fairly recently (about last November) bought a Sim free, unlocked, Xiaomi Mi A2 phone. My Vodafone contract period had ended, so I downsized my contract to Sim only, and put the same Vf sim card into my new (unlocked) Xiaomi phone.
Now I want to use the same handset with an O2 payg sim, and it says that the phone is locked to Vodafone!
Why on earth do Vodafone think they can do that - it's my phone, purchased from another retailer, nothing to do with my sim only contract!
So I looked online at Vodafone, and found the online form to apply for a free NUC (network unlock code). Great, I thought. Fill in the form - drop down list to enter your handset manufacturer - no Xiaomi! Come on guys, Xiaomi are not that new, and are getting very popular.
Yesterday I phoned customer services - after endless delays on hold, listening to the most awful repetitive music, I finally got through to a very nice woman, but who seemed to barely understand or speak english. I explained that I couldn't fill in the online NUC form because my phone model wasn't listed. I gave her the exact model of my phone, and IMEI number. She said that the NUC team would send me a code by email.
Shortly after speaking to this nice lady I did get an email - apparently automated, because there's no way to reply - just repeating the exact same information as on the website - and including the link - to the ONLINE NUC FORM! That I can't use!
After reading about others' experiences I'm not holding out much hope for a resolution to this any time soon. Vodafone - please, if you're listening, please help!
May I ask where you purchased your Xiaomi Mi A2 phone from please ?
Also had you tried other sim cards from other networks in it when first owned to ensure it was unlocked to all network's ?
Was it listed as unlocked to all networks in the small print etc ?
I ask because Vodafone wouldn't be able to lock an already unlocked phone to their network.
It used to happen with iPhones years ago where it would lock to the first sim inserted into it if the iPhone was purchased from Carphone Warehouse as an example.
Have you used your sim card in the phone for at least 30 days and then used this unlocking page > unlock-code-request.
I wish you all the best with this situation.
Current Phone > Samsung Note 9 _512Gb > Model: SM-N960 Hybrid-Duos.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.
If you purchased the phone as SIM free and unlocked, it won't be locked to the network.
I am wondering if this is SIM PIN you need for the SIM change and not the unlock code. The default PIN would be 0000, if you enter it incorrectly 3 times the phone will ask for a PUK code, obtainable from Customer Services or live chat.
If this works and you are able to get into the phone, I would suggest doing a factory reset to clear any stored codes in the phone and set the phone up with your O2 SIM.
Please see this link: PUK code