[Nexus One Q&A, Help & Troubleshooting] blisk:
I'm getting RMA'ed, again, for the same issue plus not being able to register on the network after I get the x with four blank bars. I get either "error searching for networks" or "sim is invalid on this network". It's since fixed itself... somehow but i was without any phone service despite trying everything (calling tmobile for network outages, hard reset, reseating the sim card, other stuff) for about 3-4 hours twice now... and a few shorter time periods (1-3 minutes of those errors) before that.

Possible solutions.. which I've only done after it started working again but didn't cause anything bad to happen. So far have been without any of the previous issues, plus my original long standing issue of not being able to send texts, get any sort of data, or call (I call it the up arrow without a down arrow problem where the phone is trying to connect but doesn't receive anything back). These solutions were originally posted here:

http://www.google.com/support/forum/p/android/thread?tid=3f4ad5ffac46bf92&hl=en

http://www.google.com/support/forum/p/android/thread?tid=47dc569ca9667ba7&hl=en

They're also solutions I've come up with for my issue as well and seemed to have worked for a couple of others.

It's as follows:

*#*#4636#*#* and selecting phone information. Press the menu button and select the proper band. I'm guessing if you're in canada on rogers which uses the same band as AT&T US you'd select USA band. Anyway after selecting it should re register on the network.. close out of there. Turn off and turn on the phone and see how that goes. For people in other regions I have no idea what to properly select but it was posted on one of the threads on the google forums that it should be "automatic". Check with your provider and/or google tech support.

Another thing I've seen is again in the *#*#4636#*#* menu under phone information put in (THIS NUMBER IS ONLY VALID FOR TMOBILE USA) under SMSC 12063130004 and press update. Call your provider for your number. Restart the phone.

Less likely solutions (in order of descending likeliness to help you in your particular case):

Hard reset and/or verify the APN settings for your carrier. I've been told non factory default APN values for tmobile over the phone (google support) so I don't know if verifying them or asking for them will help or cause more issues but sometimes there's apps the mess with the APN values which could cause some of your issues. So while this has a high potential to help, it's easy to screw up. Hard resetting the phone will give you factory default values (I'm 99% certain but from what I remember). Changing them from non factory default isn't a terrible thing however call your provider rather than google tech support for the values.

Calling up and asking to add/verify the IMEI number to the network (i never did this until recently although it didn't fix my issue right when they added it anyway) and/or doing a network cancellation might help. Unlikely but if you' RMA'ed once they'll have the wrong one on file and could cause issues depending on the network.

Replacing the SIM card. Takes literally 30 seconds. They ask for account info at the store, they scan the new one.. and give it to you. You pop it in and go. If you have an older SIM card this DEFINITELY could be the issue. Its unlikely its defective but it's always worth ruling that out.

Let me know if any of these help.