When a mobile device uses a roaming SIM, the process of selecting a local network to connect to is typically managed by the device itself. Here's how it works and what you need to know:
Who Decides the Network?
- In many cases, the device - not the SIM or provider - makes the decision about which local network to connect to.
- Some mobile networks do use a “preferred PLMN list” (Public Land Mobile Network list), which can guide the device to connect to certain networks first. However, not all providers do this.
- For example, KPN does not use steering or a preferred list. Instead, the device is free to choose from any available network in the area.
How the Device Chooses a Network
When there’s no preferred network list:
- The device scans for available networks in the area.
- It usually attempts to reconnect to the last network it successfully used. For instance, if the device last connected to EE, it may automatically try to reconnect to EE again.
- If the network is still available and allows registration, the device stays connected - even if a different network nearby (like Vodafone or O2) has a stronger signal.
Why the Device Might Not Switch Automatically
- Roaming devices often “stick” to the last known network. This is called “network persistence.”
- Simply restarting the device or refreshing the SIM doesn’t always force it to re-evaluate the available networks.
- As a result, the device might remain connected to a weaker or problematic network, even when better options are available.
What Can Be Done to Help a Device Switch Networks?
- The most effective solution is to manually select a network on the device. This allows the user to choose from the list of available networks and pick the best one for that location.
- However, manual network selection is not always possible—for example, in devices without a user interface or in systems where access is limited.
- Additionally, attempting a SIM refresh, physically adding or removing SIM bars, testing the SIM in a different device, or restarting the device (power cycles) might yield positive results, though success is not guaranteed.
Summary
- The device controls network selection unless the SIM is guided by a preferred list (which some operators like KPN do not use).
- Previous connection history heavily influences future selections.
- If a device remains on a weak network, manual selection is ideal, though not always feasible.
- Understanding this behaviour can help troubleshoot connectivity issues with roaming SIMs, especially in areas with multiple available networks.