Yes. According to ORO.FTL.225 (a), a time period with a start and end time, during which a crew member must be available to be contacted to receive an assignment, must be defined.
A crew member may, during the standby period, be notified that standby has ceased. CS FTL.1.225 establishes further conditions.
GM1 CS FTL.1.225 (a) explains that a minimum rest period according to ORO.FTL.235 should be provided after the notification of the revised end of the standby period.