SIP Communicator is an audio/video Internet phone and instant messenger that supports instant messaging and telephony protocols such as SIP, Jabber, AIM, ICQ, MSN Messanger, Yahoo! Messenger and IRC.
SIP Communicator is a Java cross platform project still in alpha release with some bugs but has some great features that worth to keep in mind. Can be used as Skype alternative but it is focused on privacy and security.
You can subscribe for free at iptel.org in order to obtain a SIP address.
Once you have been registered at iptel.org launch sip communicator and press File -> Add new account. As network select iptel.org and give your registered user name and password. From Tools - > Options -> ZRTP you can press the 'Standard' button and then press save. By doing this you can make encrypted phone calls to other users which use the same system.
Sip communicator is an excellent XMPP (Jabber) client. You can easily register at jabber.org in order to get a free jabber account. Once registered go again to sip communicator and press File -> Add new account. As network now select jabber.org and give your registered user name and password. By using Jabber you can enable OTR (Off the Record Messagging) in order to make encrypted conversations with your friends. Let's see how :
Go to Tools - > Options and press the Off the record messaging option. At account select your jabber account (iptel does not support OTR) and press the generate button in order to generate a fingerprint. Check Enable private messaging, automatically initiate private messaging and require private messaging. Now you are ready to talk privately with your jabber friends. Just start messaging and press the lock button on the chat window.
With a few words use iptel in order to make private phone calls using the ZRTP technology and use jabber in order to chat privately using the OTR technology. Below you can find some useful links.
Of course Sip communicator can be used normally as SIP Client in order make national and international low cost calls or even free calls. For example if you use www.voipbusterpro.com as provider all you have to do is to find its SIP configuration. Going here : www.voipbusterpro.com/en/sipp.html we see :
Software configuration
General
SIP port : 5060
Registrar : sip.voipbusterpro.com
Proxy server : sip.voipbusterpro.com
Outbound proxy server : leave empty
Account name : your VoipBuster Pro username
Password : your VoipBuster Pro password
Display name/number : your VoipBuster Pro username or voipnumber
Stunserver (option) : stun.voipbusterpro.com (not needed for sip communicator)
Now go to File -> Add new account. As network now select SIP and press the advanced button. Now press the advanced tab and fill the information. As display name and authorization name use the user name of voipbusterpro. Now go back to Summary tab and give your password. Now you should be able to make phone calls.
Please remember that Sip communicator is still in alpha and has some bugs but it is updated daily. Download always the latest version.