What I've learned.
I live in the US, but work in frequently in Austria so I use a pre-paid sim through T-Mobile when in Europe. T-mobile Austria's 3G network for pre-paid phones is insanely expensive if used on a phone; about 10X the price for a USB 3G modem. (Seriously T-mobile - why? Nobody is ever going to use this at your price so why bother?)
I set my APN for T-Mobile Austria before I knew the cost, got a E20 refill which was gone in the space of a few hours without ever using the phone. I called T-Mobile who explained that the money had all gone on my data connection. I figured I needed to change the APN setting so that the phone only connected to the network when I absolutely needed it, so I changed the APN setting from gprsinternet to #gprsinternet.
Another E20 got flushed down the toilet. It seems I was still connected to the network. I deleted the APN all together and then flushed another E20 down the toilet. (Every time I checked my network status, I seemed to be out of range of the network, so I assumed I was ok - bad assumption!)
So; the upshot is that once you set an APN, there's no un-setting it. Somehow the phone caches this information and won't let it go - ever! The work around is to do a full reset. I pulled the battery out, then started the phone up. The phone no longer connects to the T-Mobile network.
One thing that aggravated me was that when I was in an area where I had both a wi-fi connection and a 3G connection, the phone always used the 3G connection exclusively. Once I burned through the money I couldn't use my email (browser worked though) over the wi-fi until I had removed the battery.Two things need to be fixed here:
1) wi-fi needs to take precedence over 3G when the phone is connected to both, or at least have some way to let the user decide.
2) There needs to be a switch to enable the user to easily connect disconnect to the mobile network at will.
This has been an expensive experiment!