B727 CREW; AFTER LNDG GFK; TURNED OFF ONTO A PARALLEL RWY BACK TO THE TERMINAL AND CROSSED AN ACTIVE RWY.

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ON THE FLT FROM MEM DIRECT GFK; MY FO WAS MONITORING 121.5 ON THE #1 RADIO; AND WRITING DOWN THE FREQS ON HIS APCH CHART CLIPBOARD (BECAUSE OF TERRORIST ATTACKS). THE B727-100'S HAVE ONLY 1 HEAD FOR EACH RADIO; AND THE VOLUME IS ON THE UPPER KNOB. MY FO HAD A NOISE REDUCING HEAD SET AND NUMEROUS TIMES AFTER CHANGING FREQS THE VOLUME FOR THE RADIO WOULD BE TURNED WAY DOWN; AND I WOULD TURN IT BACK UP. AFTER AN UNEVENTFUL FLT AND LNDG RWY 35L GFK; TWR TOLD US TO CONTACT GND ON 124.57. I SAW MY FO TAKE HIS PEN OUT AND START TO WRITE IT DOWN. DURING THIS TIME WE WERE DOING OUR ACFT LNDG CHKS. I HEARD MY FO CALL GND EXITING TXWY A2 ONTO TXWY A. LATER I HEARD HIM CALL AGAIN 'ON TXWY A TO THE RAMP.' I DID NOT HEAR GND CALL US SO I TURNED UP THE VOLUME ON THE #2 RADIO AND NOTICED THE FREQ WAS 121.75. I ASKED MY FO IF HE HEARD GND ANSWER US AND HE SAID NO. I STOPPED AND CALLED THE TWR AND HE SAID THEY HAD TO SEND A LIGHT ACFT AROUND ON RWY 8 BECAUSE THEY COULDN'T TALK TO US. NEITHER ATIS NOR NOTAMS GAVE THE GND FREQ AS 124.57; WHICH COULD HAVE PREVENTED THE INCIDENT; THEY DID PUT IT ON THE NEXT ATIS. ALSO; ATIS NEVER MENTIONED LNDG ACFT ON RWY 8. 121.75 IS THE PUBLISHED GND FREQ.