Jim will know more about this than anyone else around the forum. However, I "think" cellphone service when using the ship's cellular signal system is limited to voice and texting and Global email and GlobalAccess are not available.
But wouldn't the cellular be taking its signal directly from the satellite?
No. As I understand it all cellular service goes through the same system provided by the cruise line's contractor - Wireless Maritime Services. WMS has contracts with the various wireless providers which is where the billing occurs, and limits on service imposed.