As you can see in the Google Map we had just crossed the Dutch - Belgian border. The landscape is dotted with beautiful cast-iron Belgian border markers (from the 1850's) that have been lovingly and meticulously documented.
And the antennas of the Dutch mobile network have been mapped just as meticulously, though probably less lovingly. A similar mapping has been done for the Belgian mobile network. It is interesting to note how the electromagnetic and geographic borders do *not* overlap.
There is a name for this phenomenon and it has been researched in depth by the Dutch telecommunication authorities. It is called "unwanted roaming". The conclusions from this research are:
Unwanted roaming occurs when you physically move a mobile phone to a location on Dutch soil where the Dutch network is absent and / or where the signal level of the Dutch network falls below a minimum usable value. This will make the unit look for a different network than its default network. When a usable signal is present from a foreign network, the device will connect to this network.
At all measurement locations signals from mobile telephone networks from Germany and Belgium are present. But at all locations further than 15 km from the frontier, the Dutch providers have more than sufficient field strength, so the risk of roaming is virtually nil.
The results of measurements and statistical research have shown that in the worst case - on the frontier - only limited unwanted roaming occurs. The biggest risk of roaming occurs for UMTS networks.
The measurements do not suggest that operators in Belgium or Germany have tuned their network to forcibly roam Dutch users to these foreign networks.
Onderzoek ongewenste roaming in de grensstreken, Provincie Limburg, Agentschap Telecom, 16 februari 2009