Originally Posted by vanbyriver

It's the same principle whether 135 or 121. On call is not defined by regs. If your are required to answer phone and report, then you are not in rest. If you are not in rest, then you cannot look back to find required rest period. There is no limit on how long you can be "on call." However, if you are on call for 24 hours, upon being given a duty assignment, you would have to be placed into rest. It is the dirty secret in 135 that many carriers blatently ignore this limitation. This is very fertile ground for an knowlegeable attorney post-accident, particularly for punitive damages where fatigue can be shown to have contributed to accident. Review the FAA Chief Legal Counsel Interpretations on the subject.

Best luck and fly safe