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, and some phones are sold as "works on any carrier" only because they depend on an rSIM adapter, which buyers price differently from a genuine unlock. 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.