I will give you some background info on the situation. On an Android phone, there is only 1 signal indicator at any given time. I believe this is because Android was originally on a GSM phone where there is only one signal indicator. However, on CDMA phones (like Verizon) there are actually two different signals. Your 1X data/voice signal, and your EVDO (3G) signal. Due to limitations in Verizon's network, you cannot be connected EVDO and 1X/voice at the same time. And 3G is not used for traditional voice services. So when you make a phone call, you are disconnected from your 3G data service, and you revert back to 2G for voice. Generally, your 1X/voice signal will be stronger than your 3G data signal.
So what does this mean for you? You are using a Verizon Network Extender. It is meant to give you better voice coverage in areas that it is lacking. However, the Verizon Network Extender ONLY does 1X/voice service, not 3G. So your phone is still connected to a local tower when its on 3G and not in a call, but when you make a phone call, it uses the extender, since it offers the best signal.
This explains why you get a strong signal in a phone call, but once you hang up, it drops to a low signal. The low signal you are seeing is your 3G data signal. However, the 3G data signal is still strong enough to give a far better data speed than 2G.
So pretty much, what you are seeing is normal, and there is not much you can do about it. But its a pretty common mistake people on make on CDMA Android phones, due to the way Android shows you signal information.
The "I have no clue what your talking about" version:
Your phone is using 3G when its not on call. When its in a call, it uses 2G. The Verizon network Extender only extends 2G coverage. Your phone is using 3G when its not on call.
Something to try out:
Turn off wifi. Then you will see a 3G indicator (granted you live in an area with enough coverage). Then make a phone call. Your 3G indicator will disappear. This is because you lose your data connection while your in a call.