IPhone :: 4 Use Be Connected To WiFi And Still Be Connected To 3G Network At The Same Time?

My iPhone is on the Bell network, along with 3 other iPhones in my household. We have WiFi and the phone indicate that they are connected to the WiFi when we are in the house. However, my bill from Bell indicates several instances where the phones (all 4 at different times) are connected to the 3G network for 24+ hours at a time. Additionally, it shows the phone connected twice during that time period (as though there were 2 different sessions at exactly the same time, but with differing amount of data connected)

The explaination that Bell gives me is that there are 'minimized' applications still working on 3G even though the phone says it is on WiFi, and that each of these opens its own 'connection'My understanding of cell technology is that the phone can only access one method of Internet connectivity at a time, and that it can be explained as a 'pipe' or 'straw' carrying connectivity and all of the data. I have never heard of a phone that can access two different types at the same time, nor one that uses separate 'sessions' at the same time of either type of connectivity.

I would assume that I turn on my phone in the morning and I am in my house, it indicates WiFi, so I can browse the web, collect my email and update my Facebook on that 'session' that lasts 4 hours until I leave the house. Now my 3G kicks in and the email that I still have open now collects through the opened 3G session, as well as the page I left open downloading something.When I get to work and enter my WiFi area there, the session would then collect my email through WiFi again, and my download as well.

Info:
iPhone 4, iOS 5.1