Although this is frustrating, no carrier can guarantee coverage indoors. I know it sounds silly, although at the end of the day you can have the best and newest yet your phone is just a radio that needs to pick up a signal. "Marginal Coverage" exists everywhere.....even Chicago, Las Vegas, Miami, etc. This is why a signal booster through Verizon is not free considering they cannot guarantee service indoors. You can switch providers and see if anything changes, although when I travel the US I rarely have problems... if any.
WHERE AND HOW DOES VERIZON WIRELESS SERVICE WORK?
Wireless devices use radio transmissions, so unfortunately you can't get Service if your device isn't in range of a transmission signal. And please be aware that even within your Coverage Area, many things can affect the availability and quality of your Service, including network capacity, your device, terrain, buildings, foliage and weather.