A phone that works abroad is not connected to its usual network. It is being carried by a local operator under a commercial agreement, and that arrangement determines the cost.
The visited network carries the traffic
On arrival the phone searches for available networks and registers with one that has a roaming agreement with the home operator.
The home operator is contacted to confirm the subscriber is valid and to establish what services are permitted, which is why service appears a minute or two after landing.
Calls, messages and data then run over the visited network's infrastructure, and the visited operator bills the home operator at a wholesale rate.
Retail pricing is a separate decision
What the customer pays bears no fixed relationship to the wholesale rate, since the home operator sets its retail roaming prices independently.
Regional agreements between countries have capped or eliminated retail roaming charges within certain blocs, which is why travel inside those areas costs nothing extra.
Outside those areas, prices vary enormously between operators for the same destination, and daily passes, bundles and per-megabyte rates coexist confusingly.
Data behaves differently from calls
Data sessions can be routed back to the home network or broken out locally, and the choice affects latency and which services appear available.
Background activity continues whether or not the phone is being used, so updates and synchronisation can consume an allowance without any deliberate use.
This is why roaming bills are dominated by data rather than calls, and why disabling background refresh has an outsized effect on the total.
Local alternatives break the chain
A local prepaid subscription buys service directly from the visited operator at domestic prices, which is almost always cheaper for extended stays.
Embedded subscriber modules allow a second profile to be loaded onto a compatible phone without changing anything physical, so the home number stays reachable.
Registration requirements differ by country, and some require identity documents at purchase, which is worth checking before relying on it for arrival.
What to check before departing
The three things that matter are whether the destination is inside the home operator's included zone, what happens if it is not, and whether any spending cap applies.
Caps that suspend data after a threshold are protective but can cut off navigation at an inconvenient moment, so knowing how to lift one matters.
Calls received while abroad may also be charged in some arrangements, which surprises travellers who assume incoming calls are free everywhere.