Could be just network related, and hopefully will clear up soon if none of the above helps.
It could be some network issue.
First, try to reboot your phone and see if it works
If not, try to reseat your sim card, power down the phone, then take the sim out for a minute before you put it back and power it up again
If it still no receiving incoming calls , try to change the network to 3G Only (or WCDMA only) and see if it helps