I suggest you try Thirdlane PBX (http://www.thirdlane.com).
I did a large call center using Thirdlane.
you will find the best any easy configuration for agents, ringing groups and queue.
It has vtigercrm CRM also.
I tried elastix (http://www.elastix.org) and I added the call center module on it but it didn't work properly.
About the phones I suggest you use free soft phones like SJ Phone, X-lite.