Manual: http://www.3cx.com/manual/3CXPhoneSystemManual.pdf

Chapter 6. Specifically sections "Step 2: Add the VoIP provider account in 3CX Phone System" and "DID’s and VoIP providers" are most relevant.

NOTE: XCast does not use authentication for sending calls to a 3CX. You must not enter Authentication information and must also select your DID's on the Source ID tab.

• Start to add a Voip Provider.
• Select Generic SIP Trunk.
• Click Next.
• Fill in the Voip Provider section with the information you were given in email from XCast.
• Click Next.
• Enter the External Number you are provided through XCast.
• Skip the Authentication fields.
• Enter the Max Simultaneous Calls with the number of ports you have been allocated from XCast.
• Click Next.
• The rest of the details are specific to your use...

Once the Voip Provider Wizard is complete, go back and edit the new Generic SIP Trunk you just created and do the following:

• Select the DID tab.
• Enter all the DID's given to you from XCast.
• Select the Source ID tab.
• Enable the option Source Identification by DID.
• Select the To: User field to match the DID's against.
• Specify the DID numbers. The option Add Mask allows you to add a mask that covers a range of DID numbers. Add DID number will bring up a dialog allowing you to select from DID numbers that you have already inputted in the DID tab of the VoIP provider.