We connect to the internet using ADSL installed on the existing phone system (copper wires) and via several ISPs. Every subscriber, from his ADSL modem/router, can log in to any ISP (other than his ISP) if he has a username/password to the other ISP.
The following example clarifies the issue:
I have a 1-Mbps subscription with ISP-1, so I have my username/password to login to my account using my ADSL modem installed on my phone line. My friend has a 2-Mbps subscription with ISP-2, so he has his username/password to login to his account using his ADSL modem installed on his phone line.
When my friend is logged out, and when I replace my username/password, in my modem, with my friend's one, my modem will log in to my friend's account and takes his bandwidth 2-Mbps on ISP-2 !!!, Noting that the rest of the modem settings are the same.
Typically, I think that every ISP should have a separate DSLAM in the phone company, but in this case, it doesn't seem like that. I need to understand How it works and what the topology used in this case.