Voip telephony simplifies your infrastructure, lowers TCO, while adding advanced features
previously offered 
only by high end systems.
A single infrastructure for your phone and Internet/LAN (no need for a separate landline
infrastructure) unlimited extensions, voice messages to mail, 
IVR, and the ability to use
the same extension carrying the same number wherever you go 
are some of the features
you get with Voip.

Depending on the customer preference and budget, commercial or well known open source
systems can be used.