A method for handling packet-switched data transmissions by a mobile station with a first subscriber identity card coupled to a first radio frequency (RF) module and a second subscriber identity card coupled to a second RF module, executed by an adapter of the mobile station, is provided. An uplink Internet protocol (IP) packet is received. Information regarding at least one of the received packet and hardware status of the mobile station is collected. One subscriber identity card is determined from the first and second subscriber identity cards according to the collected information. The received packet is transmitted to an IP network via the RF module corresponding to the determined subscriber identity card.