What open-source SIP clients (or libraries) there are that could be freely extended by developers?

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开发者_开发百科:

Some of the features we plan:

• shared contacts (if agent A creates a new contact, agent B can use it)
• central call log (all agents see all calls)
• in case the call is answered, a customer profile pops up (simply a browser is opened/brought to foreground: http://server.com/profile/{callerid})

Are there any open-source SIP clients (or libraries) which could be extended to support those features?

The only open source C# sipstack that I know of is lumisoft.

You can find a UA example here: http://www.lumisoft.ee/lswww/download/downloads/examples/

(UserAgent = client)

For C# there's sipwiz's stack: sipsorcery.

(The licence looks like the 3-clause BSD licence.)

You can use SipekSDK (which uses a wrapper for PJSIP C language stack, which is a solid and well documented one) http://code.google.com/p/sipeksdk/