Certifications for devices?

I read that if my device (Arduino Uno and cellular shield) is not certified (by PTCRB), then my carrier can shut it down. I know certification cost a lot.
My question is: How does the carrier know if it is certified or not? They have never asked me before.
Thanks

Counter question: can't you just buy a certified shield?

wvmarle:
Counter question: can't you just buy a certified shield?

Yes you can, but that doesn't make the whole device PTCRB certified.

The carrier can read your IMEI.

aarg:
The carrier can read your IMEI.

Good to know. Is that read from the module, or the modem shield?