Thanks for your reply!
Actually, I've visited the recomended page before your suggestion, but it was no very helpfull (quite informative, but not objective solutions)
I found some objective information to my modem specifically on
this page. I've followed that steps and the modem worked. I needed just to run setserial with some parameters.
The modem was on ttyS4! The only problem is that at each new boot I need to run de command again. I want to know how can I make this change definitive.