How a call reaches your mobile
A mobile network is a patchwork of cells, and each cell is the area that one mast, or cell tower, covers by radio. To reach your mobile, the operator looks up the group of cells your phone was last seen in, has every mast in that group call out for it, and connects the call through the mast your phone answers. Today your voice crosses the air in small packets over 4G and 5G, and the older 2G and 3G networks are closing.
Purple: the call and the parts it usesKey
- Purple: the call and the parts it uses
- Dashed: the radio link through the air
- One tile: the area one mast covers
Your mobile is switched on · the register last saw it in area B
Press a button, or click your phone to have someone call it.Press a button, or tap your phone to have someone call it.
Same steps in both. Your phone and the way your voice crosses the air changed.
Anatomy of a call to a mobile: a caller's house, the mobile operator's switch and register, and a town as a board of tiles, one tile for each mast. Tile A belongs to area A, tiles B1 and B2 to area B. The switch shows the same network in the 1990s. The drawing is not to scale.
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Follow one call
Press Play to watch the call, or scroll down and the drawing follows you one step at a time. Try it, under the drawing, makes your own call.Press Play to watch the call, or Next to go one step at a time. Try it, under the drawing, makes your own call.
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Step 1 of 5
Someone dials your mobile number
A friend at home picks up a desk phone and dials your mobile number. The number tells their phone company which mobile operator you belong to, so it hands the call over. The call arrives at your operator's switch, the computer that connects its calls. Nothing in your number says where you are.
Why does the call go to your operator first, and not to a mast near you?
Because only your operator knows where your phone is. The standard for a call to a mobile names the switch that first receives it the gateway. Its first job is to ask your operator's register for a way to reach you.
The register asks the part of the network that serves your area for a temporary number, the roaming number, and hands it back. The gateway uses it to send the call on to the switch near you.
Sources: ETSI TS 123 018: basic call handling in mobile networks (2022)
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Step 2 of 5
The register knows which area you are in
The switch asks the register, a database with one entry for every phone the operator serves. Your entry does not name the mast you are near. It names the location area your phone was last seen in, a group of neighbouring cells. In the drawing that is area B, two tiles of the board.
How does the register know where your phone is?
Your phone tells it. Every mast belongs to an area, and when your phone moves into a new one, it sends the network a short message to say so. The standard calls this a location update, and the register for your phone the home location register.
Inside one area your phone stays quiet, however many masts it passes. It also checks in at set times when it does not move. That lets the network tell a phone that is still switched on from one that has gone silent.
Sources: ETSI TS 123 012: location management in mobile networks (2022)
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Step 3 of 5
Every mast in that area calls out for your phone
The register says area B, not which mast. So the switch tells every mast in area B to call your phone, and both send out a short radio message with your phone's identity on it. Engineers call this paging. The mast on tile A stays quiet, because your phone is not in its area.
Why not keep track of the exact mast instead?
Then your phone would have to report every time it crossed from one cell into the next. Each report costs battery and radio time, on every phone that moves.
Areas are the trade. Your phone reports only when it enters a new area, and in return the network calls out in every cell of that area when a call comes in. 4G and 5G work the same way with tracking areas. The network pages your phone in every cell of the tracking areas it is registered in.
Sources: ETSI TS 123 012: location management in mobile networks (2022); ETSI TS 123 401: the 4G network and its paging (2023)
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Step 4 of 5
Your phone answers the nearest mast, and rings
Your phone hears its identity and answers through the mast it hears best, here the one on your own tile. The network now knows your cell. It connects your friend's call through that mast, and your phone rings. You pick up, and your voice crosses the air to the mast, then runs by cable back through the switch to your friend's desk phone. In the 1990s your phone sent its voice in one time slot out of eight on a radio channel. Today it sends it in small packets, as an internet call does.
Look inside The radio link
- 1Your voice, in packets
- 2The voice coming back
- 3The radio link
50 packets a second carry your voice each way, 20 ms of sound in each.
- 1Your time slot
- 2Seven other calls
- 3One frame, 4.6 ms
- 4The radio channel
8 calls share one GSM radio channel 200 kHz wide, each in its own time slot.
How far from the mast can your phone be?
On GSM, the 2G standard, about 35 km. Your phone talks in short bursts, each in its own time slot, and the first burst it sends has spare time around it. The standard sizes that spare time for a phone up to 35 km from the mast.
In a town the masts stand much closer. Each mast can carry only so many calls at once, so a busy place gets smaller cells, each with its own mast, and the same radio channels come back a few cells away. Bell Labs engineers called these two ideas cell splitting and frequency reuse.
Sources: ETSI TS 145 001: the GSM radio path (2022); Bell System Technical Journal: the cellular concept (1979)
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Step 5 of 5
You walk on, and the call moves to the next mast
While you talk, your phone keeps measuring how well it hears its own mast and the masts around it, and sends the numbers to the network. As you walk onto the next tile, the mast on the roof of the flats becomes the stronger one. The network sets the call up through that mast and tells your phone to switch over. The call carries on. Engineers call this a handover.
Who decides when to switch masts, your phone or the network?
The network. Your phone measures, and it reports all the time. In GSM, the base station controller and the switch choose the new channel, from the phone's measurements of its own mast and the masts next to it.
Look inside: the radio link in the 1990s and today
The way a call finds you has not changed: the register, the call-out in your area, the nearest mast. What changed is your phone and how your voice crosses the air.
The air between your phone and the mast, laid out as a track. In the 1990s your call had a time slot of its own; today it sends packets. Point at a part, or tap it, to see what it does.
1990s, 2G
Today, 4G and 5G
The radio link
- 1Your time slot
- 2Seven other calls
- 3One frame, 4.6 ms
- 4The radio channel
8 calls share one GSM radio channel 200 kHz wide, each in its own time slot.
The radio link
- 1Your voice, in packets
- 2The voice coming back
- 3The radio link
50 packets a second carry your voice each way, 20 ms of sound in each.
How your voice crosses the airETSI TS 145 001; GSMA IR.92
1 slot in 8
A burst of 0.577 ms in every 4.6 ms, on a channel 200 kHz wide
Packets
Voice over 4G, called VoLTE, sends your voice as internet packets
How often your voice is measuredETSI TS 146 010; ETSI TS 126 171; GSMA IR.92
8,000 times a second
GSM's full rate speech coder: 13,000 bits a second
16,000 times a second
The wideband speech coder every phone for 4G calls must support
Where the network calls out for your phoneETSI TS 123 018; ETSI TS 123 401
Location area
Every cell in the area your phone last reported
Tracking area
Every cell in the tracking areas your phone is registered in
When 2G and 3G close
A phone that can only use 2G or 3G stops working when its network closes. Each country, and often each operator, sets its own date.
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2028
Belgium's three mobile operators will shut their 2G networks in the course of 2028 and 2029. Their 3G networks closed in 2025.
Belgian Institute for Postal Services and Telecommunications: 2G networks shutdown (2026)
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2030
In Luxembourg the last 2G networks are expected to run until about 2030. POST finished closing its 3G network in October 2022.
Chambre des Députés: answer to parliamentary question 1899 on the 2G and 3G switch-off (2025)
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2033
In the United Kingdom, the mobile operators will offer no 2G or 3G network after 2033 at the latest.
GOV.UK: joint statement on the sunsetting of 2G and 3G networks (2021)
How it came to be
- 1946
A phone in the car
The Bell System opens a public mobile phone service for cars in St. Louis, with three radio channels for the whole city.
Bell System Technical Journal: Advanced Mobile Phone Service, introduction and background (1979)
- 1947
Small cells
D. H. Ring of Bell Laboratories describes small cells that reuse the same radio channels, so many more people can call at once. The work stays unpublished.
Bell System Technical Journal: Advanced Mobile Phone Service, introduction and background (1979)
- 1968
Find the phone by its area
Kin-ichiro Araki of Japan's public telephone company introduces small cells with wide-area location registration, so the network keeps track of the area each phone is in.
- 1979
Cells as hexagons
Bell Labs sets out the cellular concept in full. It draws cells as hexagons, because a hexagonal layout covers an area with fewer masts than squares or triangles do.
- 1991
The first GSM call
Former Finnish prime minister Harri Holkeri calls the deputy mayor of Tampere over Radiolinja's new GSM network. The call lasts just over three minutes.
- 2021
3G off in 29 minutes
Deutsche Telekom switches off its 3G network in Germany at the end of June, 22,000 antennas in 29 minutes, and moves the frequencies to 4G and 5G.
Where Voxbi fits
Voxbi is a cloud phone system hosted in the European Union. When someone calls your office number, it can ring your mobile too, through the Voxbi app over mobile data or Wi-Fi, or through a Voxbi SIM for fixed-mobile convergence over the mobile network, and the calls you make from it show your business number.
See it in Voxbi