no signal even with visible signal bars

[12vdc]

If it has a signal then its not a dead spot. What does the signal indicate, E,H,LTE? From your description if 2G cant connect then it could be a network pollution causing calls to discard voice/sms traffic. Report the incident to the telco as this could be a network outage that is left unnoticed. If the building has the pico cells installed then the more reason to suspect a network outage. Sometimes those boosters are useless if ur boosting a signal from a "down" network. Those pico cells are better as it is connected to the main base station capable of automatically allocating channels and handing-off signals to nearby cell sites.

[kisshmet]

Check your cell phone setting if it prioritizes mobile data