A communications device capable of simultaneously operating in a dual-band CDMA communications system having a cellular and a PCS CDMA communications systems and a dual-band GSM communications system having a GSM 900 and a GSM 1800 communications systems includes a radio signal transceiver module, a CDMA module, a GSM module, a first CDMA filtering unit, a second CDMA filtering unit, a first GSM filtering unit, and a second GSM filtering unit. The first CDMA filtering unit, the second CDMA filtering unit, the first GSM filtering unit, and the second GSM filtering unit are utilized for discarding signals in an overlapping frequency band between the dual-band CDMA and the dual-band GSM communications systems.