There are those operators, and those pilots who work for those operators, who do not require the pilot to answer the phone. The pilot has 24 hours a day of rest. If the pilot chooses to go to work that day, and if the pilot wishes to make money that day, he may choose to answer the phone; one call doesn't interrupt rest (in all cases), as established long ago. If the pilot does not answer his phone, another pilot may be called. The pilot might accept an assignment in thirty minutes, or in thirty hours, as may be; another long-used method, and for some it works. It also meets legal requirements.
It is incorrect to assume that one cannot be on call and at rest; one can, depending on how the rest and call is arranged. If one does not have a present duty to act for the company, one is not on duty, and it does not interrupt rest.