The maximum calls the system can accept is 24. Now, your implementation may limit this even further because, if you are following best practice, you have some ports dedicated for outcalling and MWI (which do not accept ingress calls). Whether you do this or not I am not sure. By default, this type of separation is not enforced.
With respect to failover behavior on the secondary Unity system when it happens to catch a call, I recommend reading through this document:
http://www.cisco.com/en/US/partner/docs/voice_ip_comm/unity/5x/failover/guide/ex/5xcufge040.html#wp1168710
It explains the behavior when the secondary Unity server handles a call in good detail. The summary is that when the primary and secondary nodes are both on line, one of of two things could happen when the secondary Unity host receives a call:
1. A failover is initiatied and the primary Unity ports are shutdown while control of services are transferred to the secondary node. This is configurable and you can disable it.
2. If auto-failover when the secondary node receives a call is disabled, then the secondary will answer the call and transfer it to the next available port on the primary Unity system. If, the primary Unity system is 100% utilized (i.e. all ports configured to answer calls are actively handling calls) then the secondary system will NOT answer the call (i.e. fast busy).
HTH. Read the doc, it is a good one.
Regards,
Bill
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