The clog is somewhere underneath the house in the cast iron pipe.

Snake the line. If no clean out access, remove the closest toilet.

Or down the vejnt stack from the roof if its safe to get on your roof.

I've had luck with a rubber bladder which goes on the end of a garden hose. Get a bladder the right size for the pipe. You feed the hose into a cleanout in the basement. Feed it in as far as possible so there is not a lot of air between the bladder and the clog. Attach the hose to a faucet and turn on the water. The bladder expands to fit the pipe tightly and a small hole in the end of the bladder lets out water to put pressure on the clog.

Here's an example:
http://www.homedepot.com/h_d1/N-5yc1v/R-100569152/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053

Do it right. Snake it with 3/4" snake.