Did she actually tell you what the model of the phone was or just say it was a "smartphone"?

Its highly unlikely she knows, they can check imei numbers of calls made from a sim card (and towers connected to etc.). However, this wouldnt be real time to a customer service agent you called in to and getting information on an IMEI number they haven't sold is also extremely difficult.

I suspect she just assumed it was a smartphone because it didn't have the web'n'walk application installed and its rare for someone to query data usage on an unlocked cheap phone from a non home-network (from experience).

Now they likely have a block of iPhone imei's, obtained on request, and filter these out. I wouldn't worry really about them filtering out N900 imei's unless they do pick up the phone and put it on a special data plan.