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This diagram shows how a call travels from your mobile phone through the radio access network, into the core network, across to the destination carrier, and finally to the recipient's phone.
UE
User Equipment
Your mobile phone. Connects to the nearest cell tower using LTE/5G radio signals.
eNB / gNB
Base Station
The cell tower. Handles radio communication with your phone. eNB for 4G LTE, gNB for 5G NR.
MSC
Mobile Switching Centre
The brain of the voice network. Routes calls, handles handovers, and connects to other networks.
HLR / HSS
Home Location Register
A database storing subscriber info — your number, services, and current location (which MSC you're attached to).
PSTN/IMS
Core Network
The interconnect between carriers. 2G/3G uses SS7/PSTN. 4G/5G uses IMS (IP Multimedia Subsystem) for VoLTE.
VoLTE
Voice over LTE
Modern 4G/5G calls travel as VoIP over the data network using IMS/SIP — not the old circuit-switched method.