eSIM & mobile
jargon, decoded.

An APN (Access Point Name) is the network address your phone uses to connect to your carrier's mobile data.

Bill shock is when your monthly mobile bill is unexpectedly huge — usually from roaming charges, premium calls or overage fees.

A black spot is a location where one or more mobile networks have no signal — common in rural areas, basements and lift shafts.

A carrier-locked phone is tied to one network and won't accept SIMs from other providers until it's unlocked.

A coverage map shows where a mobile network has signal, usually with separate layers for 4G, 5G, indoor and outdoor.

A data cap is the maximum amount of mobile data your plan includes each month before charges or throttling kick in.

A dual SIM phone can use two SIM cards (or one physical SIM and one eSIM) at the same time.

The EID (eSIM Identifier) is a unique 32-digit number that identifies the eSIM chip in your device.

An embedded SIM (eSIM) is a programmable chip soldered into a phone that replaces the traditional removable SIM card.

An eSIM QR code is the activation token a carrier sends you — your phone scans it to download the eSIM profile.

EU Roaming used to be free for UK users under "Roam Like at Home" — most UK networks reintroduced fees after Brexit.

A Fair Use Policy (FUP) caps how much of your data allowance you can use while roaming, even on "unlimited" plans.

UK networks are switching off 3G in 2024-2025. All voice and data is moving to 4G/5G with VoLTE.

4G LTE is the fourth generation of mobile network technology — the workhorse standard underpinning UK mobile data since 2012.

5G is the fifth generation of mobile network technology, offering faster speeds and lower latency than 4G.

The ICCID is the unique 19-20 digit serial number printed on a SIM card (or written to an eSIM profile).

The IMEI is the unique 15-digit identifier for the physical phone or device — not the SIM.

The IMSI is a unique 15-digit number stored on your SIM that identifies you as a subscriber to a specific carrier.

A Mobile Network Operator (MNO) is a carrier that owns and operates its own physical network infrastructure.

A multi-network SIM holds profiles for several mobile networks and switches between them automatically to keep you on the strongest signal.

A Mobile Virtual Network Operator (MVNO) is a carrier that resells service on another network it doesn't own.

Network switching is when a multi-network SIM moves your phone from one underlying carrier network to another to keep you on the strongest signal.

Pay As You Go (PAYG) lets you top up credit and pay only for what you use — no monthly fee.

A PLMN (Public Land Mobile Network) is the technical identifier for a mobile network — every UK carrier has its own.

Roaming is using your mobile service on a foreign network when you travel abroad.

Signal strength is usually measured in dBm — a negative number where closer to zero means stronger signal.

A SIM-only plan gives you airtime (calls, SMS, data) without a phone bundled in — you bring your own device.

SM-DP+ is the server a carrier runs to deliver eSIM profiles to your phone over the internet.

Spectrum is the range of radio frequencies a network is licensed to use — different bands have different propagation properties.

Tethering — also called Hotspot — lets you share your phone's mobile data with another device over Wi-Fi, Bluetooth or USB.

Throttling is when your carrier deliberately slows your connection speed — usually after you hit a fair use limit.

"Unlimited data" plans don't cap your monthly usage in GB — but most still have a Fair Use Policy.

An unlocked phone accepts SIMs from any carrier — no software restriction tying it to one network.

VoLTE (Voice over LTE) lets you make voice calls over a 4G or 5G data connection instead of legacy 2G/3G voice channels.

Wi-Fi Calling (VoWiFi) lets your phone make and receive normal calls over a Wi-Fi connection instead of a cellular signal.