Meetme Extension to DID mapping

I am excited to learn more about Asterisk capabilities.
I have setup a VoIP SIP trunk, extensions for Telephony Adaptors and Meetme conference room.
I would like to map one of the DID numbers (from SIP trunk) to extension that is setup for Meetme Conference bridge.

would anybody please help me in this regard? Please let me know what command lines can be inserted in the extensions.conf file to allow outsider access the the meetme extension through DID number.
Currently insider can only access it through Meetme extension 2255.

I will really appreciate your kind help.
Dhiman Deb Chowdhury