SIM not supported in an unlocked T-Mobile iPhone


    I bought an iPhone 6s from someone online.  They told me it was from T-Mobile and it's unlocked.  But when I inserted a SIM card from another carrier, I got the SIM not supported error message.  Could someone please help to check whether the phone is truly unlocked? I used a couple of IMEI services and the lock status shows unknown.  Thanks!

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