A phone sold in one country can perform poorly on a network in another despite being the same model. Frequency allocation, decided nationally, is the reason.
Spectrum is allocated country by country
Each government licenses portions of the radio spectrum to operators, and the portions available differ because of history and existing users such as broadcasting.
Standards bodies assign band numbers to these ranges so equipment can describe what it supports, but no band is universal.
An operator builds its network on the spectrum it holds, and a phone must support those specific bands to use it.
Hardware sets the limit
Support requires filters, amplifiers and antenna paths tuned for the frequency, all of which occupy physical space inside the phone.
Manufacturers therefore produce regional variants, including bands that matter in the target market and omitting others to control cost and layout.
Because the limitation is physical, it cannot be addressed with a software update, which is why band support is a permanent property of a device.
Partial support degrades quietly
A phone missing a band will usually still connect, by falling back to another band or to an older technology.
The user sees a working connection at reduced speed, or coverage that fails in places where other phones work, with nothing indicating the cause.
This is why imported handsets acquire a reputation for poor reception on particular networks despite being identical in every other respect.
Carrier aggregation multiplies the requirement
Networks combine several bands simultaneously to raise throughput, and a phone must support each band involved to benefit.
Supporting a band individually does not guarantee support for every combination, since specific pairings must be implemented and certified.
This is a common reason two phones on the same network in the same place report different speeds despite similar specifications.
Checking before buying is straightforward
Operators publish the bands they use, and manufacturers publish the bands each model variant supports, including the specific variant code.
Comparing the two lists takes a few minutes and identifies the mismatch that would otherwise appear only after the phone is in use.
Attention to the low band used for wide-area coverage and the mid band carrying most capacity matters most, since those determine everyday experience.
The very high frequency bands attract attention in marketing but cover small areas, so their absence changes far less than a missing mid band would.