If you've recently activated your phone and it's not yet able to make phone calls or use the Internet, performing a Profile Update from the device forces the phone to request its device configuration from the network which should get you up and running.
On your phone:
Settings > Network Profile Update > Update Profile > Done
Once you've selected "Update Profile," all you need to do is wait until the update completes and then reboot your phone.
If you have a problem completing the update, make sure you're in an area with good, strong signal; then disable Wi-Fi and enable data.