Generally if you call in and clear DPA you can get a password change done for your own online account, at least my cellphone provider does that (O2.) In the absence of being able to ring 789 you can call 0345 600 0789 from any other line. If you clear DPA details like name, address balance/etc they should help you get that sorted.
There is also some chat online options for the mobile side of things again. Visit the Contact Us page: http://www.virginmedia.com/customers/contact/ -> Mobile. Then I guess 'My Account app'. It is account related. Click the "Chat Online" button. Use a desktop/laptop to access the page, do so between 8AM to 8PM weekday wise. If it says all agents are busy, reload the page every 3 minutes and check again.
- - - - - - Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.