If you're an AT&T customer and you see the SIM not provisioned MM#2 error message on your phone, it means that your SIM card isn't registered or activated with AT&T. This can happen for a few reasons:
1. You just got a new AT&T phone and inserted your old SIM card from another carrier. To use your new phone, you'll need to get a new SIM card from AT&T (which you can do at any AT&T store or online).
2. You recently switched to AT&T from another carrier, but didn't properly activate your new SIM card. To fix this, simply call AT&T's customer service line at 1-800-331-0500 and they'll help you out.
3. There's an issue with AT&T's activation system, which is preventing your SIM card from being registered correctly. In this case, the best thing to do is again call customer service at 1-800-331-0500 and they should be able to help you troubleshoot the problem.