I've lived in the Napa, California area (94559) for a little over 2 years.  I've noticed that anytime I am in the south Napa area (populated by residential and a lot of retail) I have to give up on trying to use my cell data.  I either don't use my phone or if I'm lucky enough to jump on a wifi then I can use it.  When does Verizon plan to upgrade cell coverage in areas that are populated, such as this?