A phone crossing a border sometimes attaches to a network with poor coverage while a stronger one is available. The selection is made from a list rather than by signal strength alone.
The handset follows a preferred network list
Home operators write a priority list of partner networks in each country, stored on the SIM, and the phone works down that list before considering anything else.
The ordering reflects wholesale agreements between operators rather than coverage quality, so the preferred network may not be the best one where the user is standing.
The phone only moves to a lower-priority network when the preferred one is genuinely unavailable, not merely weaker.
Border regions cause the visible failures
Radio signals do not stop at borders, and a mast on one side can be the strongest signal for some distance into the other.
A phone that attaches to a network across the border is roaming in that country for billing purposes, regardless of where its owner physically is.
This matters most where roaming terms differ sharply between neighbouring countries, and it is the reason travellers occasionally receive a welcome message for a country they have not entered.
Automatic reselection is deliberately slow
Once attached, a handset stays on a network until the signal degrades past a threshold, because constant switching would drain the battery and drop connections.
The thresholds are set conservatively, so a phone can hold onto a weak network long after a better one has become available.
Toggling flight mode forces a fresh scan and selection, which is why that step resolves the problem more often than any setting change.
Data roaming and voice can behave differently
Data may attach through one network while calls and messages route through arrangements that differ, particularly where the partner lacks a full agreement.
Certain services, including some calling features, depend on the visited network supporting them, and support varies between countries and operators.
This is why a phone abroad can have working data and unreliable calls, or the reverse, without any fault in the handset.
What manual selection changes
Switching network selection to manual lets the user choose from the networks the phone can see, overriding the priority list.
The chosen network must still be a partner of the home operator, otherwise the phone will fail to register and show no service at all.
Manual selection also stops automatic fallback, so it needs undoing before moving on, which is a common cause of losing service in the next region.