There are times when you're better off calling the local sewer department first.

In San Jose, if you see evidence that your house's main drain has backed up and you have a cleanout within 5' of the sidewalk, you're better off calling the city first before calling a plumber -- the sewer department will snake the "lateral" pipe between the cleanout and the main sewer line under the street for free.

The one time we used this the response time was very quick (in line with the 30 minute response time they cite on their website).