Open Source IP PBX vs Tradition PBX

Comparing the flexibility, extensibility, and power of an Open Source VoIP Solution with a traditional PBX is somewhat akin to comparing a personal computer with a calculator. Traditional PBXes do what they do very well -- but that is all that they do. In contrast, an Open Source VoIP Solution is a very malleable tool that can address a wide range of needs:

Standard PBX functionality such

call forward (busy, no answer, unconditional)
caller ID
call transfer (announced and blind)
three-way calling
visual call waiting
do not disturb
music on hold, music on transfer
call parking
call pickup
Direct Inward Dialling (DID)
multiple call appearances
speed dial
group call pickup

Advanced PBX functions such as:

automated attendant with multiple menu levels, time-of-day changes, and staff directory
dial-by-name office directory
voice mail with message waiting indicator
multiple extension ring
remote (off-premise) extensions
flexible/dynamic dial plans
conference rooms with up to fifty participants
Automatic Call Distribution (ACD, aka Agent Call Queueing)

All these features are just the beginning, this is one smart investment which you can make.