Communication is bidirectional so there is no need for the downlink to be more powerful than the uplink: the phone must be able to send data to the satellite us

Communication is bidirectional so there is no need for the downlink to be more powerful than the uplink: the phone must be able to send data to the satellite using its own antenna and battery. However in theory a more powerful downlink could send data down to the phone faster than the phone sends it up to the satellite.