SIM & Mobile Glossary

This glossary covers the core terms you will encounter when comparing SIM plans, setting up an eSIM, or switching carriers. Definitions are ordered by topic: start with the basics, then move to setup, network technology, and number portability. When you hit an unfamiliar term in any SimFinder guide, come back here.

Core Terms: SIM, eSIM, and Carrier Types

SIM

Subscriber Identity Module. A circuit that stores your carrier identity (IMSI — International Mobile Subscriber Identity), authentication key (Ki), and network credentials. Every mobile device needs a valid SIM to authenticate with a carrier’s network. Standardised as part of GSM in the early 1990s.

Physical SIM cards come in three current sizes defined by ETSI TS 102 221:

Most smartphones released in the past five years use Nano-SIM (4FF).

eSIM

Embedded SIM. An eUICC chip soldered into the device at manufacture. Instead of inserting a physical card, you download a carrier profile over the internet — usually by scanning a QR code or through a carrier app. Governed by the GSMA SGP.22 specification for consumer devices.

Key behaviours under SGP.22:

• Multiple profiles can be stored on one eUICC. The storage limit depends on the chip’s memory, not a fixed specification cap.
• SGP.22 v2.x: Only one profile can be active at a time.
• SGP.22 v3.0+: MEP (Multiple Enabled Profiles) allows multiple profiles to be active simultaneously — the technology that enables eSIM-only dual-SIM phones.

US-specific note: iPhones sold in the US from the iPhone 14 onwards are eSIM-only — they have no physical SIM slot. This includes the iPhone 14, 15, 16, 16e, and iPhone 17 series models sold in the US.

iSIM

Integrated SIM. The SIM function is built directly into the device’s main chipset (SoC) rather than on a separate eUICC chip. This allows even thinner and lower-power devices. The GSMA SGP.32 specification (published May 2023) covers IoT-focused iSIM deployments. Consumer iSIM devices are an emerging category.

MNO

Mobile Network Operator. A carrier that holds a government radio-spectrum licence and owns its own network infrastructure — towers, core network, and radio equipment. All mobile traffic in any given country ultimately runs through an MNO’s network. Examples: AT&T, T-Mobile, Verizon (US); EE, Three, O2, Vodafone (UK); Telstra, Optus, TPG (AU).

MVNO

Mobile Virtual Network Operator. A carrier that sells mobile service without owning its own radio network. MVNOs buy wholesale network access from an MNO and resell it under their own brand. The ITU defines an MVNO as “an operator that provides mobile services without holding a government radio-spectrum licence.”

As of 2022, more than 1,986 active MVNOs operated across 80+ countries (GSMA Intelligence). MVNOs typically offer lower prices than the MNO they ride on, because they carry less overhead. However, they share capacity with the MNO’s primary customers, which can affect speeds during peak hours.

Sub-brand

A lower-price service operated by an MNO’s subsidiary or affiliate, using the parent carrier’s own network. Sub-brands sit between an MNO and an independent MVNO in terms of both price and quality. Because they use the full parent network, they generally deliver more consistent performance than independent MVNOs. Examples: Visible (Verizon), Cricket (AT&T), SMARTY (Three UK).

Prepaid

A service model where you pay before you use. You purchase a fixed amount of data, calls, or time credit upfront. When the allowance or expiry date runs out, service stops until you top up. Most travel eSIMs use a prepaid model.

Postpaid

A service model where you pay after you use. The carrier bills you at the end of each month based on your plan or usage. The standard contract form in most developed markets.

Setup and Configuration Terms

APN

Access Point Name. The gateway identifier that tells your device how to connect to mobile data. Defined in 3GPP TS 23.003. Every data session requires a valid APN. In 5G systems the equivalent term is DNN (Data Network Name), defined in 3GPP TS 23.003 Clause 9A.

You usually do not need to set the APN manually — most carriers push the correct settings automatically. Manual configuration is sometimes required with MVNOs or travel SIMs when automatic setup does not trigger.

eSIM Profile

The bundle of carrier data downloaded to an eUICC: network authentication credentials, APN settings, phone number, and related configuration. Profiles are delivered from the carrier’s SM-DP+ server to your device’s LPA (Local Profile Assistant). Once downloaded, you activate, deactivate, or delete profiles from your phone’s settings.

QR Code Setup

The most common way to install an eSIM. The carrier generates a single-use QR code containing the SM-DP+ server address and an activation code. You scan it with your phone’s camera (or through the carrier settings screen) to download and activate the profile. Each QR code can only be used once.

SM-DP+

Subscription Manager Data Preparation+. The server infrastructure that creates, stores, and delivers eSIM profiles to devices. Defined in GSMA SGP.22. Operated by the carrier or eSIM provider. When you scan an eSIM QR code, your phone connects to the SM-DP+ server to retrieve your profile.

SIM PIN and PUK

PIN (Personal Identification Number): A 4–8 digit code that prevents unauthorised use of your SIM. After three consecutive incorrect PIN entries the SIM locks.

PUK (PIN Unblocking Key): An 8-digit code used to unlock a PIN-locked SIM. After ten consecutive incorrect PUK entries the SIM is permanently disabled. PUK codes are set by the carrier and cannot be changed by the user. They are printed on the SIM packaging or available from the carrier’s support team.

IMEI

International Mobile Equipment Identity. A 15-digit number that uniquely identifies a mobile device, defined in 3GPP TS 23.003. Used by carriers to identify equipment on the network and to block stolen devices. To find your IMEI: dial *#06# or go to Settings > About Phone.

Network Technology Terms

3G, 4G/LTE, and 5G

The successive generations of mobile network standards.

3G — Third generation. Standards include WCDMA and CDMA2000. Several markets have completed 3G shutdown: Australia finished its 3G switchoff in 2024, meaning any device that cannot make VoLTE calls cannot place voice calls on Australian networks.

4G / LTE — Fourth generation. LTE (Long-Term Evolution) was standardised in 3GPP Release 8, frozen in December 2008. The dominant network type in most developed markets today.

5G — Fifth generation. Standardised from 3GPP Release 15 onwards (NSA version frozen December 2017; SA version frozen mid-2018). Uses Sub-6 GHz and millimetre-wave spectrum for high throughput, low latency, and massive device density.

Frequency Band

A specific portion of the radio spectrum allocated for mobile use, identified by a band number (e.g., Band 3, Band 28, n77). Carriers in different countries use different bands. When buying a device abroad or using a travel SIM, confirm that the device supports the carrier’s bands — otherwise you may get reduced speeds or no signal.

SIM Lock

A software restriction that prevents a device from working with SIM cards from carriers other than the one that sold it. Regulations on SIM lock vary by country:

• US: No unified federal requirement — carriers set their own unlock policies (typically after a device is fully paid off or after a lock period).
• UK: Ofcom banned mandatory SIM locking for new phones from December 2021.
• EU: Rules vary by member state. Some have banned SIM locking under the European Electronic Communications Code (EECC); others still permit it.
• AU: No specific regulation bans SIM locking, but in practice most devices are sold unlocked.

A SIM-free (unlocked) device works with any compatible carrier’s SIM worldwide.

Dual SIM

The ability to use two SIM cards — physical or eSIM — simultaneously on one device. Defined in GSMA TS.37. Two common implementations:

DSDS (Dual SIM Dual Standby): Both SIMs are registered to their networks and can receive calls, but only one radio transceiver is shared. An active call on SIM 1 temporarily disconnects SIM 2 from the network.

DSDA (Dual SIM Dual Active): Two separate transceivers keep both SIMs live simultaneously. Calls on either line do not interrupt the other. Consumes more power.

eSIM dual-SIM milestones:

• iPhone 13 series and later support two active eSIMs simultaneously (no physical SIM slot required on US iPhone 14+).
• Google Pixel 7 was the first Android device to support dual eSIM, launching in October 2022.

VoLTE

Voice over LTE. Carries voice calls and SMS as IP packets over the 4G LTE network using IMS (IP Multimedia Subsystem). Defined in GSMA PRD IR.92 and based on 3GPP IMS MMTel. The first commercial VoLTE service launched on 7 August 2012 (MetroPCS, USA).

VoLTE provides better call quality and allows simultaneous voice and full-speed data use. In markets where 3G has been shut down — most notably Australia — VoLTE support is mandatory for any voice calls.

VoNR

Voice over New Radio. The 5G equivalent of VoLTE, carrying voice over 5G NR (New Radio) using IMS. Standardised in 3GPP Release 15 (2018). Requires a 5G Standalone (SA) network and supports the EVS (Enhanced Voice Services) codec for higher audio quality than VoLTE.

Roaming

The ability to use your home carrier’s service on a partner carrier’s network in another country or region. Governed by bilateral roaming agreements between carriers (GSMA AA.12 standard). Data roaming is typically disabled by default and must be enabled in your device settings. Charges apply at your home carrier’s international roaming rates unless a specific roaming plan is active.

RLAH (Roam Like At Home)

The EU/EEA regulation that requires carriers to charge domestic rates for roaming within the EU and EEA (European Economic Area). In force since 15 June 2017, extended and strengthened by EU Regulation 2022/612 through 2032. Fair-use limits apply for extended stays abroad.

Note: Switzerland is not an EEA member and is not covered by RLAH, despite bordering EU countries.

Wi-Fi Calling

Making voice calls over a Wi-Fi connection instead of a cellular signal, also known as VoWiFi. Useful in areas with weak cellular coverage. Requires both device and carrier support. Availability varies significantly by carrier and country — widely available in the US and UK; less common elsewhere.

Tethering

Sharing your device’s mobile data connection with other devices. Three modes: Wi-Fi hotspot, USB, and Bluetooth. Some carriers restrict or charge extra for tethering, particularly on lower-tier or MVNO plans. Check your plan’s terms before relying on it.

Carrier Aggregation (CA)

Combining multiple frequency bands simultaneously to increase throughput. First standardised in 3GPP Release 10 (LTE-Advanced, specification frozen 2011). Release 10 allows up to five component carriers (each up to 20 MHz) for a combined bandwidth of up to 100 MHz. Used in both 4G and 5G networks.

Number Portability Terms

MNP (Mobile Number Portability)

The right to keep your phone number when switching carriers. Singapore was the first country to implement MNP in 1997. The ITU standardised the technical framework in Recommendation Q.769.1 (1999). Most developed markets now mandate MNP by law.

The process differs by country:

PAC Code (UK)

Porting Authorisation Code. The UK method for number porting. Text ‘PAC’ to 65075 from the phone number you want to keep. You receive the code by SMS immediately at no charge. The code is valid for 30 days. Give it to your new carrier to complete the port.

Frequently Asked Questions

Articles Referenced in This Glossary (Coming Soon)

The following SimFinder guides expand on topics introduced here. They are in production and will be published shortly.

• What Is a SIM Card? (coming soon)
• What Is an eSIM?
• What Is an MVNO? (coming soon)
• SIM Lock and SIM-Free Explained (coming soon)
• What Is Dual SIM?
• How to Set Up eSIM on iPhone (coming soon)
• How to Set Up eSIM on Android (coming soon)
• Mobile Number Portability Explained (coming soon)
• VoLTE and VoNR Explained (coming soon)
• APN Settings Guide (coming soon)