Tata DoCoMo has leveraged the power of Web Services while designing Billing Module which is not just a VIEW ONLY Module, but lets user interact and get things done. The following features that I saw felt are really cool,

Access to Bill Details
Activate / De-activate Value Added Services
Last 6 Months Bill Generation
Update Contact Details [Billing Address, e-mail id, etc]
Request Customer Service
and ofcourse Online Payment :-)