Hey, similar problem here from day 1 on my 64gb iphone 6 (received Friday the 19th). I can only maintain 2 or 3 bars, never a full LTE signal in areas where I usually had full. I went through the whole troubleshooting process; network reset, erasing all data & resetting, went to the Verizon store for a replacement SIM card (in case it was faulty), none of which resolved the issue. A power on/off cycle or network reset does return my LTE signal to full 5 bars but within seconds to minutes they're back to a low signal, hence less battery life from signal drain.
OP- the reason you get full bars on a call but they go back down after it ends is because you might have "enabled data only" in cellular settings. Which means on voice calls you use 3G service (full bars) and when you hang up it returns to the poor LTE signal. I experienced this firsthand, regardless I still believe I have a defective iPhone 6.
Solution- Spoke and went through the whole Verizon wireless tech process to ensure it was in fact defective, so I can qualify for the 14 day return/exchange period. Now because the iPhone's are on backorder, past the exchange period (silver 64gb ships by 10/17), my VW tech/rep had to "verify" and notate into the claim that I'd still qualify for the exchange. So it turned out like this; had to order a new replacement 64gb iPhone 6 through the tech/rep at full price (to be reimbursed after a billing cycle), get a verification text sent to me of the time & date noting the exchange being honored, and so when I recieve my new replacement iPhone, I can send my defective one back rather than send the defect back now and be phone-less for the next month.
At first I thought fedex or Verizon had damaged my iPhone in shipping (the box & packaging was horrible & shoddy upon arrival but I just thought I was being too critical). So yea.. I think more or less, a few of us are in the same boat. Hope that helps everyone.