SIP is the Session Initiation Protocol. Many VOIP phones use SIP. The other 
standard is H.323. You need a phone which talks SIP or H.323 to use VOIP.
You have the following options:

1) Get an adapter like the Linksys PAP-2T (actually developed by Sipura which 
they bought). You connect the PAP-2T to your LAN switch. You connect your 
ordinary phones (max of 2) to the PAP-2T. You can then use your ordinary phones 
to do VOIP.
2) You can get a VOIP phone from Linksys and other vendors.
3) You can get a softphone -- that is a software based telephone. I use
the Xten softphone on my Linux desktop. You can download it from
  http://www.counterpath.com/index.php?menu=download

    Now that you have your VOIP phone, you need to get a VOIP service. The Free 
World Dialup service (http://http://www.freeworlddialup.com) is OK. The great 
thing about it is that you can use any SIP phone to connect to it.  You do need 
to sign up (for free) however.

    In PHNET's case, we have deployed Asterisk (http://www.asterisk.org), as our 
soft PABX. This PABX connects us to the VOIP world (except Skype) and to the PSTN.


-- 
Bombim Cadiz
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