You should use pppd and chat scripts to connect. Here's my modified setup:
modem /dev/ttyS0 115200 crtscts
connect '/usr/sbin/chat -v -s -f /etc/ppp/peers/isp.chat' noauth asyncmap 0
ABORT "NO CARRIER"
ABORT "NO DIALTONE"
OK "AT%T21,13,1 %T21,2f,d"
ogin: <your login>
ssword: "\q<very secret password>"
If you know you need to tweak some modem init strings, this is done in the 6 and 7 lines and the actual number is sent to the modem is in 8'th line (AT - attention D - dial W - wait for the dialtone 053 - the number).
Try to dial your ISP using minicom or something like that, if you can log in using your login and password and then the terminal starts to be filled by symbols, this setup will suffice.
To dial, use
/usr/sbin/pppd call isp
isp - the name of the file in /etc/ppp/peers/
man pppd and man chat would be of great help.