Some advice you didn't ask for:
1) If you own a home, you really should own a decent plumber's snake. Often you can clear an obstruction yourself. Even if you can't, you can generally determine the approximate location of the clog.
2) Always phone the city first if there is any chance that the clog is their responsibility. Using your snake may give you a hint about the location.
3) Ask around. Almost every neighborhood has at least one person who has a long list of reasonably priced handmen. The gentleman that some of us hire for sewer issues charges about 1/2 what Roto-rooter charges. He's got good equipment, including sewer cameras. He's also honest - he questions folks over the phone and advises them to call the city first whenever that seems prudent.
4) Use the phone to get several quotes. Be certain that you understand what you will receive for a fixed price quote over the phone. For example, Roto-rooter may charge $x.xx for a basic call, but then an hourly rate of $y.yy gets tacked on if the service call on-site exceeds 45 minutes. If it is less than 45 minutes, you still pay the flat rate.
5) For all home repairs that you don't feel interested in doing yourself, have your resources lined up before the emergency occurs.
Good luck, Gideon