Our company has a very customer focused support program with the promise that all calls will be routed to an actual Agent, not a voicemail or answering service. We migrated to Zendesk Talk due to the functionality we would receive within our budget. 

We do have multiple shifts and currently do not use the Zendesk Talk IVR, we're using a 3rd party system that allows scheduling. During each shift, I have it call different numbers depending on what group I want it to ring. 1st Shift contains the most agents and they participate in an on-call rotation. I've trained our team rather than going offline to switch to  "phone" vs "browser". All agents in the 1st shift rotation is forwarded to a cell phone. This means I cannot turn off the "Agent Confirmation" prompt. Zendesk does limit the main number to only one agent unless... you add an extension. Since it's a cell phone it doesn't react to you pressing one, so for each agent I incremented the extension by 1. So if an agent steps away it will ring the cell phone in their absence or allow the on-call specialist to have x number of attempts to answer the phone. 

2nd Shift and 3rd shift numbers ring a group of individuals within that group, if they don't answer roll to 1st shift. Since everyone is offline, switched to "phone", it will ring the support cell phone.

This does create additional problems as it will create tickets under that individual. I've created work arounds with "triggers" and "automation". If ticket is created outside normal business hours (1st shift), throw to new queue and let the on-call specialist grab it. This will only occur if the agent is a part of the 1st shift group, so 2nd/3rd are not affected. 

I do believe Zendesk will continue developing and providing native functionality to support our needs over time but in the meantime we have a solution that accomplishes what is needed. 

If you have any questions/concerns, please feel free to let me know and i'll provide as much insight as possible. Again, this isn't a flawless solution to but could be used if you have a limited number of agents monitoring the phone, a solution to ensure your company has ample opportunities to answer the phone as it round robins between agents and minimize the chance a call goes to voicemail.