Option 1. Calls are routed to a phone number:
For one button you can only set one phone number.
Option 2. Calls are routed to a PBX system:
For one button you can only set one phone number. If this phone number routes to 1 person so only 1 person will receive calls but in the case of a PBX system, a phone number routes to a team / several persons so nothing will change for your agents with your current system. The queuing system will remain the same.
Option 3. Calls are routed to Zendesk:
You can add as many agents as you want to receive calls on one button. If the 1st agent doesn't takes the call, the call goes to the 2nd agent and etcetera until an agent takes the call. It's a queuing system.