Verizon iPhone 4S 16GB does not receive incoming calls

Hi all,

In December, I bought my first iPhone, and I absolutely love it. However, recently I've had (what I consider) a big issue. I am self-employed, and rely on this phone to make money from customers calling me inquiring about items I have in my store, or items they will pay a finders fee to locate.

In the past week, I've had two people say they cannot get in touch with me. One, a customer, and the other, my mother, who is in the hospital. Both say they tried multiple times, and the phone would just keep ringing, and would never get to voicemail. I looked at my missed calls, and I had no notifications of a missed call, nor any that showed up in my log. Both have confirmed they called my exact cell #. What gives? Has anyone else had this issue? I'd take it to Verizon (or would an Apple store be better?) but since this is an isolated event, I doubt it would "reveal itsellf" when the issue will attempt to be replicated in-store.

Could anyone give me some guidance or advice on what would be ideal to do with an issue such as this?