You really should follow a howto like the one above first. This way you will have more of an idea what's going on and then if you have questions you could get more detailed help.
However I'm not using agetty and recommend mgetty since I'm using it.
I can also receive faxes on the same line and they can be emailed to you as they are received.
It's not complicated at all.
Here is my options file which I have added some dns stuff and wins server to it...
# > cat /etc/ppp/options
and /etc/inittab entry which answers the phone. You have other options here so see man mgetty..
S0:2345:respawn:/sbin/mgetty -s 57600 -x 3 ttyS0
My method of starting the login in /etc/mgetty+sendfax/login.config...
/AutoPPP/ - a_ppp /usr/sbin/pppd auth -chap +pap login asyncmap 0 192.168.1.101:192.168.1.201
You could also support chap if you wish.
The method I prefer using requires that the users allowed to login are added to the pap-secrets file.
# client server secret IP addresses
david * "" 192.168.0.201
Don't put your password in the file