Very common occurrence when I was with Sprint, which prompted me to return the phone.
If you call Sprint they will tell you one of the following
- to backup ,erase, restore.
- They will re-provision something which requires deactivation / reactivation, wait 24 hours.
- Nothing wrong with the towers in your area, wait 24 hours.
- Someone will call you back (they never do)
- Call Apple or go the Apple store, something is wrong with your phone.
After doing this dance in the end you will still get the 3G or o.
So unless you are in an area with LTE I would suggest returning it within the 14 day window.
I switched to Verizon and it costs me $12.00 more per month with usable LTE.