I agree that you should not spend more than a few additional hours trying to free the cable. If the line is that bad it really should be repaired. If after a few hours of working the cable you are still hung up you really need to put a locator on the cable and dig up where it is stuck. That way you can repair the bad section of pipe and retrieve your cable in one piece.
If you have a back hoe there already ask your customer to consider an entire new line. It won't cost that much more and you won't have to worry about another section of line collapsing a week after you made a repair. Even though that wouldn't be your fault I promise most customers will scream bloody murder if you have that happen.
I assume you already know to have all utility lines located before you dig. Be careful when you are working the cable as sometimes it can tie itself into a knot just outside the cleanout and break a finger or hand in the process. Good luck and hopefully the cable breaks free and you avoid digging a big hole.
One other thing: If you must dig; Over 100 plumbers and laborers were killed in these United States last year due to trench collapses, many more were seriously injured. Don't take safe trenching and shoring measures lightly. When the ground falls you won't have time to move and rescue usually means recovery.
Work hard, Play hard, Sleep easy.