Hi, all CM models are redundant (cluster), you buy a minimum of two servers for the redundancy. You need to get in touch with cisco or a reputable partner to get an idea of model and prices, however for example the mcs-7816 is the smaller rackmount unit and would support 200 user perfectly even without redundancy.
Then if you can put an intelligent device at the branches too, this will give you the so called SRST that if the line to hub goes down, phones can still be used locally and if you have a pots line locally, it will be used. This line or lines is also always the one used for emergency calls.
However, for SRTS, you won't use the CM520 but a real router with voice, going fro the 1861 for 8 users, or the 2801 up to 24 users. The device can also termiate you WAN/MAN connections, do firewall, VPN and all the things a router does.
These are the architecture alternatives in extreme simplification. If you can read the
http://cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/6x/trunks.html
it has all the details. Note, implementation for 20 branches is not trivial and requires experience, time and a lot of attention to details. It is highly recommended you choose an experienced certified cisco partner if it is your first IPT project.
