sillejo

Are you talking IPCC as in Call Center or UCM as in CallManager?

The redundancy for UCM is in the subscribers with a hot standby for the Publisher.

For IPCC there is a high availability option for both IPCC Enterprise and Express (in 4.5 and above)

Extension Mobility does need access to the UCM Publisher because that is where all the changes are made. In CM 5+ this is designed differently since it no longer uses the Publisher Subscriber relationship of SQL server. It is now a fully replication using Informix.

DNS redundancy is ok, but that's only if your DNS servers are also High availability. The same things that can bring down a publisher/subscriber cluster can afflict a DNS server. Using IP instead of DNS is a better scenario since the SQL database replications are Dependant on IP address. It is very difficult to make a new publisher with a new IP. Much easier to have a hot standby so you can use the same IP.

What kind of redundancy are you looking for? What vulnerability are you trying to guard against that the redundant drives, NIC's, Power Supply's, and Subscriber servers do not guard against? If you are trying to guard against WAN accesibility to the Publisher, you could put the hot standby server at a separate location, assign it two IP's (the original Publisher in disable mode then a seperate one for network access) have the Publisher database backup to the hotstandby. If the real Pub goes down then turn on all services, re-enable the Pub IP on the hot standby and change your routing on the network to see the IP scheme at the new location. This all would take about 20-30 minutes if you have a DR plan in place.

Then Ext Mobility in 4.x would be fine. Again, this is served better in 5.x

__________________
Deus ex machina