"The idea is to run voice and dat on the SAME cable. That's how you save the money"
If you install a VoIP system you only need one CAT5/6 outlet saving you between £25 - £65 (depending on amount of outlets)
You need to upgrade all of your switches to support QoS and PoE, the phones are more expensive, the actual PBX (in many cases) is more expensive.
Not such a money saver in the, well pretty short term.
Also if your switch dies you lose voice and data. Normally if something breaks in the comms room you can still make and answer phone calls or compose and answer emails. With a new converged system you can do neither!
Can you believe I sell this stuff, I can't.