I'd like to know what my options are for an open-source SIP server based on Java. I'm using this for writing a client/server app, for testing.

So far I've got this one...

Cool check out this new SIP server, free and with development tools too.

https://developer.ubiquitysoftware.com

I hope this helps.

Hey here you can even get a free SIP server and development tools and all.
https://developer.ubiquitysoftware.com

Check out these guys, they've developed a framework (client/server) for networked...

JSR 125 is now officially canceled as of 14 Mar, 2006.
http://www.jcp.org/en/jsr/detail?id=125

For working with a SIP server you should check out Ubiquity's. It's based on JSR 116 (SIP Servlet)....