dropped calls

I purchased a new iPhone5 back in Septemeber from Sprint. The phone worked great and very rarely dropped a call. Last month I had it replaced because of dust issue in the camera lens. The phone that my local Apple store gave me was a "white box" phone it didn't come from retail pacakging. So, I'm assuming it's a refurb or b-stock of some sort. Ever since I replaced the phone I have been getting alot of dropped calls. 9 out 10 times it will drop within 1-2 minutes. Sometimes it would go straight to dead silence or I would hear a feedback buzz and then the call would drop. I have called Sprint several times and they give me the same run around story that they are "having technicans troubleshoot the area". Sprint also has me run power cycles and reset the network settings but alas I still get dropped calls. This happens not just where I live but even in other cities that I visit. So, is every Sprint tower I'm on "being worked on" or do I have a bad iPhone5?

-arunto