Well, the username password are binded alright, buts its really easy to know the customer account no., atleast for mtnl.You just take ip address of the user, login into the router using the ip address of the user, see the page with username password and check the html source code and there you have it, the CA no. of the user along with its phone number.You can't use the username password to surf the internet but you can do shenanigans with it, say change tariff plan etc.