Although sound quality for IP voice has historically been lacking, CODECs and processor speeds have improved to the point where you can now have clear conversations over public data networks. When you add VoIP in your private network, you get the bonus of being able to manage your own line capacities, which results in superb sound quality. SIP (Session Initiation Protocol) has become the standard signaling protocol for VoIP phones. SIP sets up and tears down sessions in two-way IP calls, as well as in multiparty voice and video conferences.
VoIP can offer some serious cost benefits. By installing IP telephony devices in your branch offices, for example, site-to-site calls can be routed over your data lines. This is especially efficient if your data lines carry high batch traffic at night but low volume during the day. Voice calls hit the PBX and then are routed over your data network--a frame relay connection, for instance--rather than going out over a voice trunk. And you bypass the PSTN and its associated charges. (LEC charges vary--VoIP can make sense if you're stuck in a high-tariff region, or if your enterprise has international branches that are already IP-connected with decent data lines.)
You can also set up new offices or build out existing offices by setting up a virtual VoIP PBX at one location to manage call volume for remote sites. It's also easy to add new IP handsets--you can just plug directly into an Ethernet port or connect to a PC and forget about hardwiring an extension.
VoIP makes moves, adds and changes less painful and less expensive. With conventional telephony equipment, you have to map extensions, program special call-handling features and activate phone sets. When users relocate, their phone configurations have to be modified and/or customized. With VoIP modules in PBXs, users can take their customized phone settings with them, even if they're just changing desks. The configuration data is tied to the user rather than a physical extension; the VoIP module just looks for the IP address of the user's phone, rather than an extension mapped to a specific port on the PBX.
Existing PBXs can be retrofitted for hybrid locations, where you keep your existing separate phone infrastructure and use VoIP for adding new hires. So if your call center expands to support 200 new agents, you can install an IP-telephony module into your PBX and run it off the site's CAT 5 Ethernet for the new hires.