zero and therefore necessarily less than the value of the loudness of the dial tone detected if the first transducer 28 properly detected a louder or the only dial tone the software branches from step 180 to step 190 and selects the first transducer as the receiving unit and the second transducer as transmitting unit the input selector 44 actuates switch s 2 to electrically couple the first transducer to the digital input 52 thereby assigning the first transducer 28 functionality of the receiving unit the drive selector 42 actuates switch s 1 to electrically couple the second transducer to the digital output 46 thereby assigning the second transducer 30 functionality of the transmitting unit if the second transducer 30 properly detected a louder or the only dial tone the software branches from step 180 to step 200 and selects the second transducer as the receiving unit and the first transducer as transmitting unit the input selector 44 actuates switch s 2 to electrically couple the second transducer 30 to the digital input 52 thereby assigning the second transducer 30 functionality of the receiving unit the drive selector 42 actuates switch si to electrically couple the first transducer 28 to the digital output 46 thereby assigning the first transducer 28 functionality of the transmitting unit as shown at step 210 the software.