Most of the time telnet is started from the inet daemon (inetd). When an incomming telnet request comes in on port 23. Port 23 gets mapped to the word 'telnet' by the /etc/services file. Once the inetd knows what service the incoming request is for it will look at the /etc/inetd.conf file (it caches it when it boots) and figure out what to start to handle the incomming requst. The inetd.conf file has an entry for telnet telling it (in a round of about way) to start the in.telnetd process.
If telnet is disabled on your system, most likely you will just have to uncomment the line in the /etc/inetd.conf file. Any changes you make to the inetd.conf file you will have to tell the inetd to reread the config file by sending it a HUP signal.
kill -HUP <pid of inetd>
Hope this answers your question.