asterisk users - Jan 2005 - call waiiting and 3 way calling

Greetings,

I recently dumped my packet8 line in favor of sixtel + asterisk/sipura
spa-1000. It's working great, but I want to enable call waiting and 3
way calling. When i am on a call and get a second call, currently
things go straight to voicemail. It looks like sixtel is passing on
the extra calls, but my asterisk box doesn't know to route it all the
way.

Is there a way to set it so that i can have call waiting, and possibly
3 way calling like packet8 does?

Thanks,
Erik