A carrier-locked phone is restricted to a specific mobile carrier's SIM cards. Inserting a SIM from a different carrier produces a “not allowed” or “invalid SIM” message; the phone refuses to connect to other networks until officially unlocked.
Carrier locks are most prevalent on US-spec stock because US carriers (AT&T, Verizon, T-Mobile) traditionally subsidised handset purchases under contract. UK, EU, and Asian markets generally have a higher proportion of unlocked retail stock from launch.
Carrier-locked stock trades at a material discount to unlocked equivalents:
- The widest discounts apply to current-generation iPhones locked to major US carriers, because demand for unlocked stock is much broader.
- Narrower discounts apply to older models, where the lock matters less to buyers in price-sensitive markets.
- No published index tracks the spread. It moves with carrier, model, unlock policy and destination market, so quote it against live comparable offers rather than a fixed percentage.
Unlock paths vary by carrier. Some carriers offer free unlock requests once contract obligations are met (proof of original purchase often required). Some unlock automatically after a set period. Third-party unlock services exist but vary in legitimacy and reliability. Wholesale buyers should always confirm lock status per IMEI before purchase, a lot sold as “unlocked” with even 5 percent locked stock is a frequent dispute pattern.