Is there a way to create a sip network with multiple telephone numbers and just one public telephone? Do I need to have a sip server for this? Can someone give me some ideas? If I need a sip sever how can I create one? I am new to this stuff.
Thanks.
Creating a SIP server will be rather difficult if you're new to it.
A better way to familiarize yourself with SIP may be to get yourself an old computer and install something like asterisk, or Trixbox community-edition, then work on developing a sip softphone.
The asterisk/trixbox server will then take care of all of your telephone extensions and incoming trunks (I recommend sipgate for incoming trunks, free with 15 simultaneous incoming calls).
There are a lot of open-source sip softphones knocking about, as well as some free .NET sip libraries that you can work with, some of which are discussed here. This will at least help in becoming more familiar with the SIP protocol.
I know this is probably not the answer you were looking for, but I do hope it helps.