Switched.net
"Unlocked" gets used loosely, but it actually refers to one specific thing: whether your phone's modem will authenticate on a SIM from a carrier other than the one that sold it to you. Here's what actually determines that.
SIM lock vs. the phone being financed
These are two separate restrictions that people mix up constantly. A SIM lock is a setting the carrier writes to your phone's modem firmware that rejects any SIM card not issued by them — that's the actual "lock." Still owing money on the phone through an installment plan is a completely different, financial restriction that has nothing to do with the SIM lock technically, even though carriers often keep a phone SIM-locked until it's paid off as a way to secure the loan. You can, in principle, have a phone that's fully paid off and still SIM-locked because nobody ever submitted the unlock request.
How a carrier actually verifies you qualify
Every phone has an IMEI — a 15-digit serial number specific to that physical device, dial *#06# to see yours. When you request an unlock, the carrier checks that IMEI against their own account records: is the device paid off, has it been active on their network for the minimum period their policy requires (commonly around 40–60 days for prepaid, longer for postpaid financed devices), and is it not reported lost or stolen. If it clears, they submit the unlock instruction to the device itself, usually over-the-air the next time it connects to their network with an eligible SIM inserted, or via an unlock code you enter manually on older phones.
Why a phone can show "no SIM restrictions" and still not work on another network
SIM-unlocked and network-compatible are not the same thing. A phone can be completely free of any carrier lock and still fail to get service on another network because of band compatibility — the physical radio hardware only supports certain frequency ranges, and carriers don't all use the same ones for the same technologies. This is the actual reason a phone bought for one country's networks sometimes gets degraded service (voice-only, no LTE/5G data) on another country's network even once fully unlocked: the SIM lock was never the problem, the antenna hardware was.
Checking before you buy a "used unlocked" phone
An IMEI blacklist check (several free tools exist, search "IMEI blacklist check") tells you whether that specific device has been reported lost, stolen, or is still tied to an active installment balance with a carrier — a used phone can pass a casual visual inspection and still fail this check, which is the single most common way people get an "unlocked" phone that later gets blocked entirely once the previous owner's carrier flags it.
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