The GUI based system-config-network in Fedora/Red Hat based distro uses wvdialer for DialUp kinda connection so perhaps this procedure(distro independent) helps
1> Open Terminal, then
$ su -
# gedit /etc/wvdial.conf
2> Paste this substituting <value> with appropriate value:
[Dialer dailup]
Modem = /dev/ttyS0 # Generally it is ttyS0
Baud = 115200
Init1 = ATZ
Init2 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
ISDN = 0
Modem Type = Analog Modem
Phone = <your number>
Username = <username>
Password = <password>
Save and Exit
3> Connect using :
# wvdial dialup
Disconnect by pressing Ctrl+C