The way I usually get this to work is using Winblows first. If you have it installed, this is the way I do it.
I use the Serial data cable versions verses the USB data cable. I have never tried the USB cable setup. If someone uses their phone with a USB data cable, point me to a site that shows how to set it up.
1. I assume you have the data cable and software for Winblows.
2. Install it under Windows and have it working for you.
3. After making a connection Open the modem log and it contains all the AT commands it used to make the connection.
4. Take those commands use them under your PPP Dailup App under Linux and set it up like you would as a generic ext modem and all should work. Sometimes you can tweak the settings and get about 10% increase.