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I bought a second hand iphone (advertised as unlocked) to use overseas. I'm a tmobile customer for years. I used the phone in the US for a short while with no problem. After I traveled , I bought a new sim card from the country I'm in now but it didn't work . It appears that the phone is still locked to tmobile. I checked the eligibility to unlock it using its IMEI, and fortunately it is. I contacted tmobile customer service to unlock it remotely. They said they need the original owner to contact them to request the unlocking!!! Like how can I reach that person?! I bought it through a seller on Amazon( whom I will report for false info!) and this seller sells hundreds of them. It's impossible to know who's the original owner:(. 
the agent offered to escalate the matter for consideration, but that's it. I'm pissed off as I cannot use the phone overseas even though it's eligible to be unlocked . Anyone went through that?