We were assuming that Apple's iPhone 3G will automatically fall back to 2G in case there is no 3G coverage for voice and data connections.
But based on complaints from many iPhone 3G users, it appears that iPhone 3G is not quick enough in switching from 3G to 2G which could result in low or no signal. So if you are observing low signal strength on your iPhone, then there is a simple but manual trick to solve the problem.
This comment over at iPhone Atlas reader, w4rmk confirms that iPhone 3G is not quick enough in switching from 3G to 2G.
W4rmk wrote "Yesterday I tried to make a phone call (new iPhone 3G). I was standing outside in the Denver, Colorado area (which has 3G) and the display said “No Signal”. I walked around waiting for a signal to come in for about 5min with no luck. I went into settings and turned off 3G and immediately got 4 bars on the edge network. So the iPhone is failing to switch to edge when there is no 3G connection."
I am a little surprised that iPhone 3G did not switch to 2G automatically.
So in case you observe low or no signal strength on your new iPhone 3G, try turning off 3G manually, this will force iPhone to switch to 2G. It might take few seconds but you could see a boost in your iPhone's signal strength as w4rmk observed. (The assumption here is that you are in an area with good 2G coverage).
iPhone 3G not switching between 3G and 2G quickly could be some technical issue as I have observed similar behavior with other 3G phones as well, lets hope Apple finds a solution, as turning 3G off manually is quite painful.