Implementation

Implementing a VoIP network is often a relatively straightforward exercise, though it is important to know which areas need to be considered. We would suggest the following key areas are looked into-

Network capacity 

Network devices (routers, switches, etc) have the capacity and QoS features required for voice trafficLAN and WAN utilisationInteraction with, and affect on, data applicationsEffectiveness of network-optimisation tools to maximize utilization through compression and latency-reduction techniques

Disaster recovery

Duplication of major PBX nodes (dependin on your chosen VoIp solution)Fallback to POTS (Plain Old Telephone System) at all or strategic locationsProtection against loss of power and power spikesEmergency use mobile phones (which could be pay-as-you-go)Emergency facilities with pre-provisioned network connectionsRe-routing via the public internet

Security (also outlined as a seperate link)

Denial of Service (DOS). Attacks via a weakly protected VoIP element could flood the network disabling data applications, or conversely a DOS attack on a data application could inhibit voice traffic.Hackers making free use of the telephone system, for example by fooled billing (making the call appear to terminate internally when, in fact, it continues externally).Snooping on network traffic, soft phones and voicemail by various devices, or by call hijacking (routing a call to a different destination).

Standards 

H.323 (Packet-based multimedia communications systems) issued by the ITUSIP (Session Initiation Protocol) issued by the IETF

There are also other, proprietary protocols in use, for example Skype.