Hi all
I am new to SIP implementation specifically using SIP, I need to use it in a Call Center, as a PBX,
in other words, to write the PBX server , and the clients call each other, all are SIP,

where to start from , links, tutorials would be enough ,
I hope I made my self clear,

urgent help please,
thanks in advance

Maybe not the urgent help you were seeking--sorry about that. So it sounds like you're looking to create a call center based on SIP-compatible devices and hardware? Couple options here. To learn more about SIP, check out SIPfoundry at http://www.sipfoundry.org/. SIPfoundry is an independent, not-for-profit organization that maintains the sipXecs project, the largest open source effort to build an IP PBX for unified communications using the Session Initiation Protocol (SIP). Their site provides a ton of useful information for working with SIP.

Of course, if you want to focus more on managing the call center than on setting up and owning its devices, might want to check out OnState, who deploys SaaS-based call centers that operate on the SIP protocol and can use existing PBX hardware as part of its solution.