I've been asked, within six months, to deploy a phone system. The exact wording was "a phone on every desk." They want inter-phone transfers, external calling, and even software IP phones for our travelling sales guy.
If this wasn't all enough I am meant to do it as cost effectively as I can. We have existing Linux and Windows servers with a fair bit of spare capacity (at a data centre).
My real problem is lack of knowledge. There seems to be information out there but I am so far out of my depth that I'm not sure what I am looking for (e.g. SIP or PSTN? SaaS or run it myself, Cisco IP Phones Vs. what?, etc).
Honest I don't even know what phones I'd buy even if I was able to rent or install a suitable phone service. Nor how that service would connect into a "real" telephone network?