Internet volume charges are separate. Undergraduate modem costs are on the margin and paid by Departments, but there is a time limit quota and students can ge kicked off. Needless to say this gets quite slow sometimes. Undergraduates can buy their way to a direct connect service by depositing finds in an Internet account. Charges are volume based with rates set to recover costs - same formula that is used to distribute costs of staff use to Departments.
Students using their Internet account can dial up for as long as they have funds.
They can buy more if they wish. The idea here is to get students to manage their use responsibly. Their use is charged to Departments. However, this is likely to change to a scheme similar to the undergraduate scheme. Internet costs will then be direct to the user, and managed by them.