It narrows them down, however there are still a ton of ways to impliment. If you want total failover redundancy you would look at more hardware costs. If you want to do something similar to what I have done I have basically a single point of failure for an "Inteligent" system that if the lines get cut from my main site all of the other sites work, however they work of off something called SRST. However depending on if you have your voice mail at the main location or at a different one that could also be effected.
I would suggest looking up some information, look at your buisness needs, and then talk to a consulting group (a few of them) to see how they would impliment, mentor, save money, and of course keep the administrative overhead to as a minimal as possible.
Due to the bandwidth requirements to keep the Quality of Service of voice traffic so that it is either 100% perfect, or has a "Cell Phone" quality data networks are impacted. This consideration needs to also be taken. If you have a site or certain things that can not be impacted, and it overrides your voice quality you will have some nice complaints. This should be something the outside vendor also should be taking into consideration.
The long and the short end of the answer - If someone is asking you to impliment this and you have no prior experience get help. If you are just researching it, continue, there are pleanty of articles out there on this system by cisco.
Hope this helps,
Kevin
