I would recommend the FOP2 (fop2.com) there is a trial version you can run indefinately with it being restricted to 15 buttons (ext, conf, trunks, parking, etc.) but for $40.US it is not resticted. there are 2 add-ons (when I originally got mine - now 5) IM Chat + 20US and Visual Voicemail+20US get everything for 80US. Want to strip the FOP2 brand out? the Whitelabel in all forms is 20US higher than above (Full blown 100US) there is a yearly maintenance for 20/yr -- I haven't gotten that in 3 years but need to start and the 3 new features that work in the newest FOP2 2.27 is a simple wallboard for Queues (10US) Set followme within FOP2 (5US) and Auto wrapup for queue agents/call centers (10us).
so depending on your needs you can get FOP2 for between 40 (basic basic) -> 145 full blown tricked out monster (add 20 for the white label version -- however I have no problem letting my people know what I use and support.
And NO I do not work for or sell for FOP2 - I am just a user who likes their product. If you need help in getting started their forum is a good place to go, or here, (I'm sure I am not the only FOP2 user here).