I got the following from a friend of mine who is wanting to use his iPhone sim card in another AT&T phone that he uses. He's encountered a few problems (detailed below). I don't have much sim card knowledge so I figured the smart folks here might be able to assist.

Here's his info below:

So, I got an iPhone2 on Friday (16gb, white). It’s pretty cool. I also have an HTC Tilt, which I also like for other reasons. In any case, I want to use both phones with the same SIM. Most people would say, “wow, that’s a no-brainer... You’re on a GSM network, just swap the SIMs.” Well, that’s what I’ve been trying, but I’m encountering some really weird issues that I can’t figure out.

Here’s the biggest.... If you swap the sims, things from a phone perspective work just fine (other than the second issue that I’ll tell you about in a minute). The biggest issue is one that is really mind boggling. Ok, enough with the suspense. I can do everything but actually surf the Web with PocketIE. Mail works fine (both ActiveSync Push and POP). The Google client works fine (I can search and get maps, etc.). Even a software update app (for Resco PocketExplorer) works (I did an update check as part of my debugging process, and actually found an update, which I was able to successfully d/l and install with NO issues). If, however, I go to PocketIE, type in “www.google.com” or any other URL for that matter, I get a status message of “Looking up” (which it successfully does), then “Opening.” After about a second or two, the status says downloading 0K, and the screen is just a blank page. I’ve seen this error before, usually when the connection goes stale, so I turned the radio off and on (flight mode). That normally fixes things. Not this time. So, I rebooted. Still no luck. Finally, I did a hard reset. Still doesn’t work! I can’t figure out what could possibly be going on that everything EXCEPT the pocket browser will successfully work.

Issue 2: Voicemail... The voicemail button on my TILT is unassigned. There’s no number stored for voicemail retrieval. When I try to go to voicemail, I get a popup box (with input) that says “voicemail is not configured on this line. please enter your voicemail number and dial again.” The input box is asking for a phone number (which, of course, I don’t know, but will try to hunt down). I’m not sure how that’s going to break things, but I may have to have ATT take visual voicemail off of my account.... I’m really not sure.

Any ideas or help would be GREATLY appreciated.

Thanks!

-KennX

Try this:

http://forum.xda-developers.com/showthread.php?t=371532

summarized issue 1 for the lazy:
If, however, I go to PocketIE, type in “www.google.com” or any other URL for that matter, I get a status message of “Looking up” (which it successfully does), then “Opening.” After about a second or two, the status says downloading 0K, and the screen is just a blank page. I’ve seen this error before, usually when the connection goes stale, so I turned the radio off and on (flight mode). That normally fixes things. Not this time. So, I rebooted. Still no luck. Finally, I did a hard reset. Still doesn’t work!