Living in Basel, Switzerland, attached to France and Germany, I can pick up many GSM networks. In my living room I picked up 9 signals. I'm sure from the roof I can pick up all of them.
I repeated the exercise what drove me mad during my vacation. Here's how I killed my iPhone's signal completely (until reboot).
1. I go into carrier preferences and selected manually E-Plus as carrier. Swisscom has roaming with them so it should work just fine.
2. After a while, I got no change into the carrier at the top left. So I go into carrier preferences again. Now I get welcomed with a popup saying "Network connection lost". Well not surprising, the signal might not be strong enough. As I'm already in the preferences I go back out and back in. Now it says it can not load my network list. This is like that until I reboot.
3. Exercise #2. I select "sunrise 3G" as a carrier. Well the iPhone lets me do that. I am 100% sure, Sunrise does NOT have roaming with Swisscom. But the iPhone lets me select it ? Yes. I got the checkmark next to it. So I went back out and everything looked fine. It still showed swisscom and some bars at the top left. Suspicious as I am
I wanted to see if it auto reselected my home carrier. So I went into carrier preferences again and voila. it switches to "No service". So far so good but now it says "Searching...." and no list appears. Never. You can wait for hours it will stay empty. The only way to get your phone back is to reboot it.
It seems to be that Apple has not well tested roaming. In USA it only works on AT&T. You can drive 5'000 miles and you will still have AT&T coverage or no coverage anyway. So why care about roaming?
In Europe we use roaming (and we get ripped of by the carriers for it). And due to SIM lock, Apple doesn't help the budget neither. In my case I can walk 10 minutes and I'm in another country (France). I can walk another 10 minutes and I'm again another country (Germany) again. At my home I can pick up signals of 8 carriers from 3 countries on which I can roam on all of them except the other carriers of Switzerland.
Coming back to my vacation spot.
There I had 4 carriers I could select from:
TDC had a strong signal. Telia a "ok" signal and Sonofon I have not seen at my vacation house and 3DK only very weak on the balcony. So I called swisscom support line asking which carrier would be best for my Laptop's 3G card (Novatel XU870). The answer was I should choose TDC as they have 3G roaming with them whereas they don't have 3G roaming with Telia despite Telia having 3G. The result was that I got GPRS speed. It was not good enough to use it for browsing or reading email even. While trying to do a wiretransfer on e-banking, I timed out on the login screen as the security key changes every minute or so and it took longer to transmit it so it was always expired. Telia supported EDGE in that region. However, my 3G card from Novatel had switched me back to TDC all the time. And there is no option to change the carrier. Apple has simply not thought of anyone wanting to do that. On the iPhone where I could at least read my mail, I had similar problems due to the experiences of switching carriers as explained above. Driving to the next village, dropped the signal for a minute or so and the guesswork started again. After a week, I fixed the problem for the laptop by manually changing the connection scripts to force a selection of carriers (using AT+COPS=...).
Its a shame a world wide company like Apple has never thought through what roaming means to non americans. I guess they have a learning curve on the mobile side.
But lets not give up hope yet and give them a chance to fix it. They fixed another problem I reported (one I'm not allowed to tell you due to NDA madness) even though they didn't told me its fixed in that new release. Could have saved me some hours of work. But what the heck.