Few Galaxy S7 owners have complained of terrible signal strength and frequent calls dropped. There seems to be a major problem for people on Sprint specifically, but some people with other companies have also had problems.
Possible solutions:
Go to settings> Mobile networks> Network mode and the CDMA / LTE switch if you are on Sprint or Verizon. If you are with a different vehicle, then Global could work better.
For some Sprint users switching to CDMA only actually resulted in a faster, more stable connection than they could get with LTE. This could be because your local tower is being upgraded. If this is the case for you then you will also want to complain to Sprint about it.