IP phones are expensive
Think about that expensive bit of gadgetry that is sitting on your desk. No,not the PC, i mean the phone.
A simple analogue phone is cheep and easy to replace should the worst happen (like a stray coffee).
Analogue phones can even be smart and show information such as caller ID (even names against internal extensions).
But other phones cost a packet. Those big networked IP phones can cost over £400 and most of the extra features are never used.
If you set up your office, you get a handful of phone lines, a hand full of numbers, a phone hub with enough ports to cover every desk and enough cheep phones to sit on even the unused desks.
If you set up your system with IP phones, its almost the same, but you leave the empty desks phoneless as you don't want to shell out that much money at the very start of your venture.
Smart desk phones a good investment? Not really in my book.