Witch is the best option in Cisco switchs and routers catalogue to a network with 50 users to implement a VOIP solution?
A 2821 with CME will support up to 52 IP phones, doing a very complete PBX, without the need for external servers or anything else.
Get ISDN for the telco lines, do not use analog lines that are alway a source of trouble. Get a CUE in AIM or NM form if users want voice mail.
For the switch, if you can afford the price, get a a 3560 or 3750 48 ports with PoE. If you can't, get two WS-CE500 24 ports, again with PoE.
Choose the phones as you like better, a good choice is the 7931 that has many buttons and a good lighted display.
That to give you an idea of what you need.
Hope this helps, please rate post if it does!