i'm in the same boat, was activated since yesterday, but still can't receive calls. it has to do with porting my number from another carrier. i read somewhere where it said it could take up to a few days to be able to receive calls to my iPhone. in the meantime i can make calls and use all the other features of the phone. i hope this process completes by tomorrow!
Be sure you have removed the plastic wrap from the phone. Several people have tried to return phones, complaining of low volume, and had not removed the protective clear plastic film. After removing the film the phone worked as normal.