In North America, cell phone performance standards and test specifications are organized by PTCRB.org for GSM cellular and CDG.org for CDMA cellular. Each of the cellular carriers may add a few additional requirements for a phone specifically qualified for their network.
Within these organizational standards are peformance requirements of the cellular antennas over the bands and the ratio of horzontal and vertical polarization.
I'm not well familiar with the requirements, but I'm not aware of provisions for diversity antennas on the cell phone or other mobil device.