Telnet is mentioned as being insecure because all communications take place in plain text. SSH is more secure since it encrypts the communication (also there are less vulnerabilities in the server side). I personally see no reason to offer a telnet service on a modern network - ssh clients are common enough and do everything a telnet session can.
It sounds like there is a bit of confusion about how you want to connect. People above have outlined two options either directly dialing up, or using the internet.
Using the internet is pretty easy - simply make sure you are connected, record your ip address:
#/sbin/ifconfig
will tell you this.
Then you start your SSH server:
#/etc/init.d/ssh start
from the remote end you then connect with any SSH client in the same way you connect to any other server.
The down side of connecting over the internet is if your ISP drops your connection. While you can set the computer to redial if this happens, you'll get a different IP address.
If you really need a reliable way to connect to your computer, then directly dialing it and having the modem answer is the best option. The trouble is it isn't easy to set up. I haven't done it in a while, but try playing around with mgetty (i think) which should give you a standard login screen when the computer answers the phone.
HTH
--e