Offtopic question

[RA]

I'm Really sorry for this   question  but there is a such situation and maybe someone could suggest something.   My Client wants VoIP based system that will operate internally in their office, receive calls coming into the office by a T-1 (they say PRI so they can use it for Internet also) and also be able to call outside the office. Internally each phone will be also operating through the VoIP protocol using their internal network. Outside calls will go through via the T-1 PRI or direct through analog lines. This we will have to suggest to them. There will not be a PABX or an EPABX or any other phone system in the office. They already have Internet connectivity into their office and each person has a PC on the network, so we have to use the same cable coming to their PC's on the LAN to extend to the phone so we save on wiring. They want to have all features such as "Hold", "Conference calling", "Voice-mail" etc. There is also no suggestion by them of any VoIP system, we can suggest any as long as we can justify the cost. They will have upto 35 telephone sets in the office and may later grow to 75 or so.   The question is : Is there anyone that has experience in VoIP, and could suggest some  technical proposal or some URL to find the answers for this ?   Thanks in advance