title: What open-source SIP clients (or libraries) there are that could be freely extended by developers?
body: <p>We need to replace our current simple SIP client (X-lite) with custom developed SIP client in order for it to have specific features we need:</p>
  
  <p>Some of the features we plan:</p>
  
  <ul>
  <li>shared contacts (if agent A creates a new contact, agent B can use it)</li>
  <li>central call log (all agents see all calls)</li>
  <li>in case the call is answered, a customer profile pops up (simply a browser is opened/brought to foreground: <a href="http://server.com/profile/" rel="nofollow">http://server.com/profile/</a>{callerid})</li>
  </ul>
  
  <p>Are there any open-source SIP clients (or libraries) which could be extended to support those features?</p>