Easiest, but not likely: your SIM is shifted or the contact continuity has degraded. Shut off phone, pull out sim and check condition of contacts on SIM. I've had to clean a really old and corroded SIM with the eraser on a pencil before. I stuck the center of the eraser over each contact pad on the SIM and gave a dozen or so back-forth twists to rub the oxide buildup away, repeating in a grid pattern to cover the entirety of the contacts. Once done, blow off any loose debris and pop the SIM back in and give it a go., Pay attention to SIM pad to phone contact alignment, particularly if you're using a cut down full size SIM, also.
IF that doesn't work, there's another common, but really annoying firmware corruption issue that occurs somewhat often:
1. Go to the phone dialer and enter *#*#4636#*#* to pull up the hidden testing menu.(4636="INFO") and the menu should pop up as soon as you press the last * key, without having to hit dial.
2. pick first menu option "Phone information"
3. The first line on the phone info menu should be IMEI. Compare it to the IMEI printed on the label under the battery.
If they don't match, it's likely that the IMEI from the menu got overwritten or corrupted somehow.
At boot, Your kernel pulls a config file from a factory-programmed, device-specific config file in the EFS folder on the root of your internal stoirage. If the EFS has bad info, the IMEI will get set to a single, specific value that is the same on every GN2. I don't remember the valyue offhand, but it's mentioned on XDA a few times in the forums.
Hopefully, you have a backup of default and proper EFS partition handy that you can restore in recovery mode.
Barring the existence of a valid backup, I think there are a few, complex methods to flash a generic stock image via ODIN, then you can go into root explorer and manually update the file(s) with the appropriate values.
I had this happen anout 2 weeks ago and freaked out because I couldn't find an EFS image backup in CWM recovery. After about a day of dinking around with no success, I realized I had a TWRP backup from when I first rooted the phone, so I restored just the EFS partition image via the advancved backup menu in the PhilZ CWM recovery.
After flashing and rebooting, the SIM got provisioned on the network, but my phone app and gvoice kept crashing. I updated my to a later, fresh version after doing a cache/dalvik/factory wipe from recovery and it's back 100% now.
Make sure you're flipping the # and * characters on both ends:
start: * before #
end: # before *