You'll be able to tell alot once you open it, with a garden hose you will quickly establish if it's plugged going in or blocked going out.
First of all the level should be aprx 12 inches from the top, if it's filled right to the top then the out going pipe/baffle is blocked. Run the water in the tank first, if the level doesn't rise above the static level then it's the main line coming in that's causing the trouble. If it fills up then its either the baffle or the field. but most likely the baffle.
Make sure they don't just pump it out without checking the inlet and outlet. Around here Roto Rooter wouldn't be the best choice, being a franchise you usually get the least experienced personnel and they will always try and sell you some other expensive service. but your local may be legit.
Good luck.