Currently there's no a simple solution for this.
The way out I see here is the following:
1. Add a FakeUser into the group, who's in fact not a real user but some email box with open access to it.
2. Set up your Calendar according to the real schedule, but in the rules make alert to be assigned to this FakeUser in case all other Group Policies are applied. This rule will also cover the case when currently there's no one on-call. See an image below
It will work in the next way:
1. Currently there's no one on-call
2. Alert is triggered
3. Application is trying to find someone on-call -> finds no-one -> according to Escalation Policy it assigns this alert to FakeUser at once. Other alerts are triggered...
4. The next employee starts working
5. He/She just needs to filter alerts by this 'FakeUser' and process them correspondingly