Three different failures wearing one costume
Cellular problems present as three distinct failures that people lump together, delaying every fix. Failure one: no signal at all, zero bars, 'No service' or 'Emergency calls only'; the phone cannot see or register on any tower, a radio, SIM or coverage problem. Failure two: signal present, calls and SMS fine, but mobile data dead; the voice registration works while the data path is broken, overwhelmingly a settings or APN problem. Failure three: everything worked yesterday and died today with no change on your part; an outage, a provisioning hiccup or a SIM failing, in that order of likelihood.
Sort yourself in thirty seconds: look at the bars, then try a call, then try a website with Wi-Fi off. Bars plus working calls plus dead websites is failure two, jump to the data section, and skip the SIM surgery entirely. No bars at all is failure one, start with the resets and SIM. Sudden death of everything is failure three, check for an outage before touching anything: your carrier's status page or social feed from Wi-Fi, or simply asking someone nearby on the same carrier, saves an hour of self-blame during a tower maintenance window.
One more triage datapoint costs nothing: does the problem follow location? A phone with no signal only at home, in one office corner, or in an elevator is experiencing coverage, physics rather than fault, and the fixes are Wi-Fi calling and patience, not settings archaeology. Signal bars are also coarse liars; the dialer code *#*#4636#*#* (in our secret codes tool) opens a status screen with the actual signal strength in dBm, where minus 90 is decent and minus 115 is barely alive, numbers that make 'bad coverage here' measurable.
The reset ladder for no-signal: cheap moves first
Radio state gets stuck, and the cheap resets clear it in escalating depth. Airplane mode on, count to ten, off: this drops and rebuilds the tower registration, and fixes a comical share of post-tunnel, post-flight and post-dead-zone hangs. Restart the phone: deeper still, resetting the baseband processor's session entirely; the classic 'restart your phone' has real engineering behind it for cellular issues specifically. These two moves resolve the majority of transient signal loss and cost ninety seconds total.
Next rung, network selection: Settings, Network and internet, SIMs (or Mobile network), find 'Automatically select network'. Toggle it off, wait for the manual scan to list carriers, select yours explicitly, then toggle automatic back on. This forces a fresh registration and repairs the state where a phone clings to a dead tower or a roaming partner it should have left. While in that menu, glance at 'Preferred network type': a phone locked to '5G only' or a similar exotic mode in a weak-5G area shows exactly these symptoms, and 'Automatic' or '4G/LTE preferred' is the sane setting.
The final software rung is the network settings reset (Settings, System, Reset options, Reset Wi-Fi, mobile and Bluetooth): it wipes every network customization back to defaults, including saved Wi-Fi and the APN edits covered below, so note your Wi-Fi passwords first. Run it when the lighter rungs fail, and before any hardware conclusion. If signal remains dead after this rung, the investigation moves physical: the SIM itself.
SIM diagnosis: the two-phone test settles everything
The SIM card is a tiny computer that corrodes, cracks and dies of old age, and the definitive diagnostic requires no tools beyond a borrowed phone: put your SIM in another phone, and another SIM in your phone. Your SIM dead in both phones convicts the SIM, carrier stores swap them free or cheap, keeping your number. Any SIM dead in your phone convicts the phone's radio or SIM reader. Both working in the swap but not in the original arrangement points back at software, revisit the reset ladder. Fifteen minutes, certain verdict.
Before or instead of the swap, service the physical card: power off, eject the tray (the pin-hole), inspect the gold contacts for scratches or dark corrosion, wipe gently with a dry lint-free cloth, and reseat firmly in the correct orientation. A SIM cut down from an older size, or a worn tray that lets the card sit loose, produces maddening intermittent 'No SIM' errors that follow bumps and pockets; a properly seated modern SIM clicks into a snug tray. 'SIM not detected' appearing and vanishing is this paragraph until proven otherwise.
eSIM changes the failure modes: nothing to reseat, but profiles can deactivate, get deleted by over-enthusiastic resets, or fail to provision on a new phone. Check Settings, SIMs for the eSIM profile's presence and active state; reactivating usually means the carrier's app or a QR they provide. Two eSIM gotchas worth naming: a factory reset can erase eSIM profiles (the reset screen asks, read it), and transferring an eSIM to a new phone is a carrier process, not a tray swap, start it before wiping the old phone, not after. Dual-SIM phones add one more classic: mobile data assigned to the SIM without a data plan; the SIMs menu shows which SIM owns data, calls and SMS separately.
Signal fine, data dead: the APN chapter
When calls work but the internet does not, the Access Point Name is the prime suspect. The APN is the address and credentials your phone uses to enter the carrier's data network; wrong, corrupted or missing APN settings produce precisely 'signal but no data'. The repair is bloodless: Settings, Network and internet, SIMs, Access Point Names, open the menu, and choose 'Reset to default'. The phone re-fetches the carrier's official configuration; toggle airplane mode after, and data returns for most sufferers of this specific pattern.
If reset-to-default does not populate a working APN (small carriers and resellers sometimes need manual entry), the carrier's website lists the exact values, name, APN string, MMS settings, under support pages titled 'APN settings for Android'. Enter them exactly; a single wrong character in the APN string kills data while leaving everything else pristine. This is also the first thing to check after inserting a new SIM from a budget operator, and after any 'no data abroad' episode: roaming needs both the roaming toggle on (Settings, SIMs, Roaming) and the home carrier's roaming provisioning, which a quick carrier chat confirms.
The remaining data-killers are quick eliminations: the data limit and warning (Settings, SIMs, Data warning and limit) silently cuts data at a threshold set years ago, a data-saver mode restricts background traffic app by app, and a VPN with a dead endpoint blackholes everything while both signal and Wi-Fi look healthy, test once with the VPN off. If data works but crawls, that is a different guide: run our internet speed test on mobile data, and if the number is real but miserable, congestion and coverage are the story, with Wi-Fi calling and band preferences as the mitigations.
Living with weak coverage, and the hardware endgame
When the diagnosis lands on 'coverage is just bad here', engineering beats suffering. Wi-Fi calling is the headline fix: Settings, Network, SIMs, Wi-Fi calling, on; calls and SMS then flow over any decent Wi-Fi, and the dead-zone house stops being a phone problem at all. Its cousin, VoLTE (HD voice), should also be on where offered; carriers have retired the old voice networks that phones fall back to without it, and a phone with VoLTE off can show service yet fail calls in modern-network areas.
Positioning and physics still matter at the margins: higher floors beat basements, windows beat interior rooms, and cases with metal or magnetic hardware measurably eat signal, test naked once before blaming the carrier. The dBm reading from the status screen turns experiments into data: walk the house, note the numbers, and put the Wi-Fi calling router where the phone lives. For rural properties, carrier femtocells and network extenders exist and work; ask support, since they are rarely advertised.
The hardware endgame is reached honestly when: any SIM fails in the phone (two-phone test), signal is dramatically worse than an identical phone side by side, the problems began after a drop or swim, or everything survives a factory reset. The cellular radio shares antennas and boards with everything else; repair economics resemble the Wi-Fi and Bluetooth chapters, weigh the quote against the phone's worth. Document before the shop visit: the dBm readings, the two-phone test outcome, and a Full Scan report give the technician a running start, and give you a baseline to verify the repair against, which is precisely how our tools are meant to be used.