GSM phones can be "locked", that is made to accept only SIM cards belonging to a specific network. Typically, this is done so that phones will work only on the network of the carrier.
SIM-locked phones are usually locked to a carrier when the carrier sells the device at a subsidized price in order to attract new subscribers to its services.
Some carriers offer a SIM unlock option to their subscribers after a certain amount of time has passed. Unlocking the phone can be done by entering a special code which is generated based on the phone's unique IMEI number.
Some third-party shops offer unauthorized unlocking of SIM-locked phones, but using their services usually voids the official warranty of the phone.
2G | 3.5mm headphone jack | 3G | 4G | 5G | 802.11
A-GPS (Assisted GPS) | A2DP (Advanced Audio Distribution Profile) | AAC (Advanced Audio Coding) | Accelerometer | Airplane mode | Alarm Clock | Alphanumeric | AMOLED display (Active-matrix organic light-emitting diode) | Analog | Android | ANT+ | Antenna | Aperture | APN (Access Point Name) | Apple AirPlay | Apple AirPlay 2 | Apple iOS | Apple iOS 10 | Apple iOS 11 | Apple iOS 12 | Apple iOS 7 | Apple iOS 8 | Apple iOS 9 | Apple Pay | aptX | Audio jack | Auto-focus | AVRCP (Audio/Video Remote Control Profile)
Bada OS | Band | Bandwidth | Bar | Base Station | Battery Charging | BeiDou Navigation Satellite System | Benchmarking | Biometrics | Bit | BlackBerry OS | BlackBerry Playbook OS | Bluetooth | bps (Bits per Second) | Brand | Broadband | Browser | Byte
Calculator | Calendar | Call alerts | Calling Plan | Camera | Capacitive Touchscreen | Car Kit | Carrier | CDMA (Code-Division Multiple Access) | CDMA2000 | Cell | Chipset | cHTML (Compact HyperText Markup Language) | CIF (Common Intermediate Format) | Clamshell | CMOS (Complementary metal-oxidesemiconductor) | Color depth | Concatenated SMS | Connected GPS | Construction | Corning Gorilla Glass | CPU (Central Processing Unit) | Crosstalk | CSTN (Color Super Twisted Nematic) | CTIA | Custom ringtones | CyanogenMod
D-Pad (Direction Pad) | Data Disclaimer | DC-HSDPA (Dual Carrier or Dual Cell High-Speed Downlink Packet Access) | Digital Zoom | Display type | DLNA (Digital Living Network Alliance) | DNSe (Digital Natural Sound engine) | Downlink | DRM (Digital Rights Management) | Dual-band | Dual-Mode | Dual-SIM | DVB-H (Digital Video Broadcasting - Handheld) | Dynamic Memory
EDGE (Enhanced Data for Global Evolution) | EDR (Enhanced Data Rate) | EGPRS | EGSM (Extended GSM) | Email client | Emoji | EMS (Enhanced Message Service) | eSIM | EV-DO | EV-DV | Exchangeable covers | External Antenna Jack | External Display
FCC (Federal Communications Commission) | Feature Phone | Femtocell | Firefox OS | Firmware | Fixed-focus | Flash Memory | Flight mode | Flip-down phone | FM Radio | FM Transmitter | Form factor | FOTA (Firmware Over-The-Air) | FPS (Frames Per Second) | Frame Error Rate | Frequency | FTP (File Transfer Protocol)
Galileo (Global Navigation Satellite System) | GB (Gigabyte) | Gbps (Gigabits per second) | Geo-tag | GLONASS (Global Navigation Satellite System) | GNSS Positioning | GPRS | GPS (Global Positioning System) | gpsONE | gpsOneXTRA Assistance technology | GPU (Graphics Processing Unit)
H.263 | H.264 | H.265 | Half-QWERTY keyboard layout | Handwriting recognition | Haptics | HEVC | Hot Spot | Hot Swap | HSCSD (High-Speed Circuit Switched Data) | HSDPA (High-Speed Downlink Packet Access) | HSDPA+ (High-Speed Downlink Packet Access Plus) | HSP (Headset Profile) | HSUPA (High-Speed Uplink Packet Access) | HTML (Hypertext Markup Language) | Hz (Hertz)
iDEN (Integrated Digital Enhanced Network) | Image Signal Processor (ISP) | IMAP (Internet Message Access Protocol) | IMEI (International Mobile Equipment Identity) | IP (Internet Protocol) | IP Ratings | IrDA (Infrared Data Association)
KB (Kilobyte) | Kbps (Kilobits per second) | Key Guard | Key Lock Switch
Land line | LCD (Liquid Crystal Display) | LED (Light-Emitting Diode) | Li-Ion (Lithium Ion) | Li-Polymer (Lithium Polymer) | LiMo OS | Linux | Location-Based Services (LBS) | Lock code | Long SMS | Long Term Evolution (LTE) | Loudspeaker
Macro | Maemo OS | Magnetometer | mAh | Mass Storage mode | MB (Megabyte) | Mbps (Megabit per second) | MeeGo OS | Megapixel | Memory card slot | Memory effect | Messaging | MHz (Megahertz) | Micro USB | microSD | microSDHC | Microsoft Exchange (Server) | MIDI (Musical Instrument Digital Interface) | MIDP (Mobile Information Device Profile) | Mil-Spec (MIL-STD) | MIMO | Mini-USB | miniSD | MMC | MMCmobile | MMS (Multimedia Messaging Service) | Mobile games | Mobile High-Definition Link (MHL) | Mobile IM (Instant Messaging) | Mobile WiMAX | Mobility DisplayPort (MyDP) | Models | Modem | Monochrome | MP3 (MPEG Layer 3) | MPEG (Motion Picture Experts Group) | MPEG-4 video | Multitouch input method | Music playback time (battery life) | Music Player
NAND Memory | Network capacity | Network coverage | NFC (Near Field Communication) | NiCd (Nickel Cadmium) | NiMH (Nickel Metal Hydride) | NOR Memory | Numeric keypad
OEM (Original Equipment Manufacturer) | OLED (Organic Light-Emitting Diode) | Optical Zoom | OS (Operating System) | OTA (Over-The-Air) | OTG
Packet Data | Pager | PC Sync | PCS (Personal Communications Service) | PDA (Personal Digital Assistant) | Percentile rank | Phone Book Access (PBA) | Phone Life Cycle | Phone Physical Attributes | Phonebook | PIM (Personal Information Manager/Management) | PIN code (Personal Identification Number) | Pixel | Pixel density (Pixels Per Inch) | Polyphonic ringtones | POP3 (Post Office Protocol) | Port | Predictive text input | Price | PTT (Push-To-Talk) | PUK Code (PIN UnlocK Code) | Push
QCIF (Quarter Common Intermediate Format) | Quad-band | Quasi-Zenith Satellite System (QZSS) | QVGA (Quarter Video Graphics Array) | QWERTY keyboard layout
RAM (Random-Access Memory) | RBDS (Radio Broadcast Data System) | RDS (Radio Data System) | Rechargeable Battery Types | Resistive touchscreen | Resolution | Ringer ID | Ringing profiles | Ringtone | Roaming | ROM (Read-Only Memory) | RS-MMC (Reduced-Size Multi Media Card) | RSA (Rural Service Area) | RSS (Rich Site Summary) | Ruggedized (Rugged)
S60 user interface | SAP (SIM Access Profile) | SAR (Specific absorption rate) | Screen protection | SD (Secure Digital) | Secondary camera | Sensors | Side Keys | SIM | SIM lock | Single-Band | Skin | Slimport | Smart Watch | Smartphone | SMIL | SMS (Short Messaging Service) | SNS (Social network service) | Soft keys | Soft Reset | Speed Dial | Stand-by time (battery life) | Stereo Speakers | Streaming Video | Stylus | Sub-QCIF | SVGA | Symbian | SyncML
Talk time (battery life) | TCP/IP | TD-SCDMA (Time Division-Synchronous Code Division Multiple Access) | TEST | Tethering | Text messaging (texting) | TFD (Thin Film Diode) | TFT (Thin Film Transistor) | Theme | To-Do list | Touchscreen | Trackball | Transflash | Transflective | Tri-band
UFS | UI (User Interface) | UIQ | UMA | UMTS | Unlocked phone | Upload | UPnP (Universal Plug and Play) | USB (Universal Serial Bus) | USB On-The-Go | USIM
VGA (Video Graphics Array) | Video call | Video Codec | Voice dialing | Voice mail | Voice memo | VoIP (Voice over Internet Protocol) | VPN (Virtual Private Network)
WAP (Wireless Application protocol) | watchOS | WCDMA(Wideband Code Division Multiple Access) | Wear OS | Wearable Technology | webOS | Wi-Fi | Windows Mobile | Windows Phone OS | Wireless email | WLAN | WMV (Windows Media Video)
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