You need read documentation for you softphones/hardphones. Usualy need add sip header Call-Info: answer-after=0.
So you plan can be
- Check if auto-answer/intercom posible with your softphone or hardphone.
- Create dialplan which will add header and call
- Transfer call to that dialplan using AMI Transfer command
For more info check this:
http://www.voip-info.org/wiki/view/Asterisk+manager+Example:+Transfer