Phone comparisons cover processors, cameras and displays in detail. The cellular modem, which determines whether the phone works where it is used, is almost never examined.
The modem is a separate processor
Cellular connectivity is handled by dedicated silicon running its own software, either integrated into the main chip or fitted as a separate component.
It manages signal processing, error correction, handover between cells and the negotiation that decides which frequencies and technologies are used.
This runs continuously whenever the phone is connected, which makes it one of the larger consumers of power in normal standby use.
Weak signal is where designs diverge
Under strong signal almost any modern modem performs adequately, so comparisons made in good conditions show little difference.
The differences appear at the edge of coverage, where sensitivity, noise handling and antenna design decide whether a usable connection exists at all.
This is why user reports about reception vary so sharply by location, and why a phone that is excellent in a city can be poor in a rural area.
Antennas compete with everything else
Antennas need space and separation from other metal, and both are scarce in a thin body full of batteries, shielding and camera assemblies.
Supporting many frequency bands requires multiple antennas, and each additional band constrains the layout further.
Hand position also matters, since a hand across an antenna absorbs signal, which is why grip affects reception on some designs more than others.
Power draw shows up as battery life
A modem that maintains a connection efficiently, and that can drop into low-power states between transmissions, extends standby time considerably.
In poor coverage the modem raises transmit power to reach a distant cell, which is why battery life falls sharply in weak-signal areas.
Battery comparisons conducted on strong local networks therefore understate the difference a modem makes for someone who travels or works in poor coverage.
Band support decides where a phone works
Networks use different frequency bands in different countries, and a phone only works on bands its hardware supports.
Regional variants of the same model often carry different band sets, which is why an imported handset can show reduced coverage on a local network.
Checking the band list against the intended network is the one specification that determines whether the rest of the phone matters at all.
Modem capability rarely appears in marketing because it is difficult to demonstrate in a shop, which is precisely why it goes uncompared.