Omnibus Telephones

Is it possible to mimic omnibus telephones in Asterisk?
Used for signalling applications, etc

These are non-busy phones that share a single speech path on off hook and can dial other extensions whilst offhook without using code prefixes, etc.
On answer these users join the conference, Ideally also off hook ring/ announce in case a phone is left off hook

Thinking of autodialling into a conference- is there a way you can just dial an extension number from within a conference to have it join?