If your AT&T phone is displaying the message SIM not provisioned, it means that your SIM card has not been properly set up for service. This can happen for a number of reasons, including if you recently switched to AT&T from another carrier or if you activated a new line of service with AT&T. In either case, you'll need to contact AT&T customer service in order to have your SIM card provisioned for service.
There are a few things that you can check before calling customer service, however. First, make sure that the SIM card itself is inserted correctly into the phone - it should fit snugly into the SIM tray and click into place. If the card is inserted correctly and the message is still appearing, try restarting your phone. Sometimes this can clear up any issues with the SIM card being recognized by the device.
If neither of those solutions works, then it's time to call AT&T customer service. They will be able to help you troubleshoot the issue and get your SIM card properly provisioned for service. In most cases, this is a quick and easy fix that can be resolved over the phone without having to go through any further hassle.