You are on the road to a great challenge my friend. You will need to be sure you understand the following:
1. QOS is not the same as COS (class of service). Class of service is another feature that you should have on your network to enable voice services.
2. VLAN (virtual local area networks). This also means that you will need to understand subnetting, routing and switch configuration. VLAN’s are necessary in the VOIP environment to give the voice traffic the environment it needs to be free from any of the broadcast storms, worms, etc. that might be present on your data network. Think of it as a virtually separate cable environment within the current cabling infrastructure.
3. Plan for growth. Will you always have 70 users? Will the system need to grow quickly? With just 70 users, you could go with a Cisco “lite” version just using a call gateway with all the “smarts” built into it without building a full CallManager cluster.
4. When you go to VLANs, think about your IP address infrastructure and how devices talk to each other. You may find some devices you didn’t know about that have to have their subnets changed to fit the VLAN model. We found devices like timeclocks and security systems that had IP addresses but nobody knew how to get to their management interfaces to change subnets or subnet masks.
5. Power to the phones, gateway & servers is critical. In the old telephone environment, power was provided by the PBX. Now, the power is provided by either the LAN switch or a power brick on each phone.
Cisco does have some good VOIP planning and implementation documentation. But, it is still in your hands to think through all of the services used by your current voice system and then think very long and hard about how this will impact your data network. The list I have above is just a start.