I've never ever set up a modem in linux but if i were to attempt to i would use the network manager rather than the command line because i wouldn't have a clue what commands to use...
Daryna > administration > network
then there should be a modem connection option highlight it and then goto properties. Now you should tick the box "enable this connection" and fill in all the data required in the box's including the username and password..
good luck with it, it's all just a guess on my behalf but this would be how i would go about doing it.