Not really a problem, mainly a question for the experts out there...
I have 3G models of the iPhone and iPad both on O2 contracts and live in a rural area, west of Glasgow and with unobstructed line of sight to the Glasgow area about 10 miles away as the crow flies. AFAIK the masts in my village area are not 3G enabled but those in the Glasgow area are.
In the house both devices show a status of 5 bars O2-UK. However, when I go into the garden the status changes to 1 bar O2-UK 3G. Once I move elsewhere in the village the status reverts to 4 bars O2-UK.
Is a weaker 3G signal overriding my strong local signal and can it be prevented?