Before paying for an unlock, verify your IMEI isn't blacklisted (lost/stolen/unpaid). A locked phone + blacklisted = worthless.
Alternatively, insert a SIM from a different carrier. If you see an error message ("Invalid SIM" or "Network locked"), your phone is locked.
iPhones work slightly differently than Androids. They don't usually use an "unlock code." Instead, the carrier whitelists your device in Apple’s database.