[Freeswitch-users] enter only existing conferences in dialplan.

I currently accept any "75xxx" numbers and transfer to a conference with pin with no problem, however currently if they mistype the number they end up creating a new conference and sit there all by their self waiting. I want to setup dialpllan entry that only allows a user to join a conference that already exists and is active. Have read thru the wiki, and the 2 FreeSWITCH books looking for examples on how to do this without creating database tables to store conference state. In reading, looks like I can use ${} and get a "conference list" and with regex patterns might be able to check for existence of valid conference ID before joining them. Does anyone have a simple approach to do this? Thanks Stephen