Same exact problem here, slightly different specs.
Various locations in New York City, generally with 4 or 5 bars.
The fact that the caller is sent to voice mail after two rings is quite serious, as it appears to the caller that you're ignoring their call when in fact you never had a chance to see it. Further, it's impossible for me to judge how many calls I'm missing due to this problem, since as far as I know, if the caller does not leave a VM I get never get any indicaiton (no missed calls logged).
Also, I've had serious problems with text message delivery to me being delayed for up to 1-2 hours, and having my own texts fail and need to be resent up to 10 times.
This has been happening for a couple of months.
FWIW I've seen Verizon iPhone users complaining about this problem on other forums.
I assumed this problem was limited to ATT because I've learned that ATT was given a higher frequency to transmit on than Verizon and the higher the frequency, the more difficult it is for the signal to penetrate buildings and obstacles.
So what happens is their tower doesn't "see" our phones and assumes they are turned off and sends the caller automatically to voicemail after 2 rings and the phone doesn't even ring the 2 times.
If the problem is most often in one place where you spend a lot of time, call them and demand they give you their MicroCell tower. It is made by Cisco and plugs into your WiFi and gives you 5 bars and seems to improve the issue. It basically turns your phone into a VOIP phone and eventually sends your signal to a working tower, or incoming calls come into the MicroCell which has 5,000 sq ft range supposedly.
I have one and am waiting to see if I get any more no rings.
But if Verizon has the problem too, then I would start to wonder if it is the iPhone after all...