As a former dispatcher at a decent sized agency, I can say that if and how quickly reports are relayed to the WFO depends on the agency you get when you dial 911. In other words, don't necessarily expect them to call the WFO right when you hang up the phone. Most agencies will relay the report fairly quickly, but keep in mind that dispatching first responders will take priority over contacting the NWS and any delay in warning during severe wx can be significant. This is especially true in smaller agencies with only two or three dispatchers working at a given time. In larger com centers, it's not such a big deal because other dispatchers can make the call to NWS while the primary LE/Fire dispatchers can roll the units.
In many states, including here in Colorado, there are direct numbers to the state/highway patrol posted on signs along the highways and interstates (*CSP here). Personally, I would suggest making note of that and using those numbers. State Patrol communications is in contact with NWS fairly consistently and they have the training and manpower to quickly relay reports to the WFO.