Either way I would do it via a Ring Group. On the mobile MiCollab client all the members can change their status form present / not present so go on duty / off as required.
If you do not have MiCollab it can also be done from a MiVB VoIP phone with a key appearance. For this though you would need to go into the office or have a Teleworker at home.
We do this the same way. Point the number at a ring group. Make anyone needing to be on call a member of the group. Then have the on call users toggle their presence in the group via the MiCollab client (mobile, desktop, or web).