From all the demos I have seen, shoretel is a very safe choice.
If there is anyone out there on more of a budget, I just ran a sample config using 3CX doing the following assumptions:
* 64 simultaneous calls with call center features (spreadsheet has the numbers to upgrade to 128 or downgrade to 32).
* 115 total phones (100 mid level and 15 basic)
* Single PRI/T1 to SIP Patton 4960 gateway (you could upgrade to one that could do multiple PRI, if needed)
* $2000 to buy a server to host a copy of windows server (also included)
* You do all the install and config (it is not that hard)
Total cost (including buying a copy of windows and a server) is under $27k. Yearly maintenance costs are are $2000 + your windows server software assurance cost.