iphone 4 no longer can hold a signal

My not-quite 2-year-old iPhone 4 (not S) now cannot hold a solid 3G signal. Whereas I used to be able to make calls from my office (concrete tilt-up, outside wall, near window) and from anywhere within my home, I now get dropped calls all the time or text messages will not go through.

I took it to Verizon and their first question was if I have an extended warranty with them or Applecare, which I don't. They then reprogrammed the cell tower sync by dialing *228, option 2.

That hasn't semed to work. My next option is a Genius Bar appointment, but I strongly doubt there will be anything they can do.

Is this a case of antenna failure?

Tony