I cant confirm if you're right or not, but I am on pay upfront and only put £10 in. I added the mobile internet for £5 a month.I think 3 realised I was using the phone as a modem and took the rest of my credit away. Otherwise i was calling folk in my pocket, i dunno.
3 dont seem to mind if u go overboard but in the t&c's it does say to you will contacted should you go over the limit.
I sit 3 weeks after my 1st £5 monthy bundle was put on, have downloaded huge files, and yet no-ones contacted me. And how could they? Its a pay upfront sim and they dont even know my gender, name, address, nothing!!!!!!!!!!!!

Works for me. Remember the t&c