How does my SIP Server can Identify the Calls?

For the example purpose you have 3 numbers on your DIDWW account, as follows:

121233334444

121233335555

121233336666

And you mapped them to your SIP server with the address 1.2.3.4.

The numbers will be forwarded to your SIP server as follows:

121233334444@1.2.3.4

121233335555@1.2.3.4

121233336666@1.2.3.4

Your server will identify each call by the called number (extension).