In our part of our company, we have non-op days.
These are days where your hours preclude you from doing a duty legally, within your roster as it stands. The company may change this if they change the preceding or following duty, however they must give us the minimum notice required of a duty change, ie 12 hours.
They cannot use the day as a standby day and if they haven't changed the duty then the only reason they can contact us is if they have a change for the following day.
However if you choose not to answer the phone then that is entirely down to you.
YD