What, exactly, happens when I call 1-800-WXBRIEF?
There is a list of Flight Service Stations in the US Chart Supplement.
The person on the phone is not a meteorologist and does not need to be. Though, just like pilots, they have been trained to be proficient on weather and weather related knowledge.
They provide the weather the same way a pilot can self-brief the weather. They utilize distributed briefings, Area Forecast Discussions, Prog Charts, other tools supplied to them by the NOAA, WPC, NWS, and AWC. Primary though, they use METARs, TAFs, PIREPs, and Winds Aloft reports at your airports of planned use and along your route of flight.
Someone will answer 24/7. Though, the person answering may not be from the closest FSS. It will be routed to the first available representative.
The same goes for inflight contact over VHF radio frequencies,