A carrier-locked phone (sometimes "network locked") is restricted by software to accept SIMs only from a specific network. If you bought a phone bundled with a contract, it may be locked to that network's SIMs.
UK rules
Since December 2021, UK carriers are required by Ofcom to ship phones unlocked — so any new contract phone from EE, O2, Three, Vodafone or their MVNOs should already work with any UK SIM.
Older phones
Phones bought before December 2021 on contract may still be locked. You can request an unlock from the original carrier — it's free and usually takes a few days.
How to check
Try inserting a SIM from a different network (or scan a Fuse eSIM QR). If you see "SIM not supported" or "Network Locked", your phone is locked. Otherwise, you're good.
See also
How Carrier Lock matters when picking a UK mobile plan
Understanding Carrier Lock helps you compare mobile providers on a like-for-like basis. Most consumer mobile pricing pages skip the technical detail — knowing the terminology means you can spot when a plan is genuinely better and when it just sounds that way. The Fuse Mobile glossary keeps definitions short and consumer-focused, no jargon-on-jargon.
Fuse Mobile is a UK multi-network data-only eSIM — it gives your phone access to all four UK mobile networks (EE, Three, Vodafone, O2) through a single profile. Where Carrier Lock is relevant to that setup, it's surfaced explicitly: pricing pages show real numbers, the coverage checker shows live per-network signal, and there's no hidden fee structure. See the plans page for the current pricing or read the multi-network eSIM explainer for the technical detail.