If mobile data is off, then it's off. No data, no internet. No mms. No google. But if you accidentally turn it on, the floodgates open and everything will try to synch...
I've had the same problem of high internet usage on one phone, with credit disappearing in consequence, despite no such usage by me. The carrier in question was unhelpful, so I am left not knowing whether it's a faulty SIM, faulty phone, or something else.
