I use WvDial to connect to the internet (their homepage is at open.nit.ca/wvdial/). Install it and follow their instructions to configure your internet connection(s). This will generate a file called /etc/wvdial.conf that contains the telephone number, username and password of your different ISPs. To connect to an ISP, simply use the command wvdial ISP (where ISP is the name of you Internet Service Provider).
The problem arises when you want to use different ISPs, as they use different DNSs. To get around this, back up /etc/resolv.conf (responsible for listing the DNS addresses), create a file /etc/resolv.conf.ISP for each of your ISPs, and use a script similar to the following to establish your connection...
Now you can establish a connection with the command connect ISP.