AN MD80 FLC ACCEPTS THE CLRNC MEANT FOR ANOTHER ACR FLT HAVING A SIMILAR SOUNDING CALL SIGN WHEN 7 MI N OF COL; NJ.

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FLT EWR-MCO. TOOK OFF EWR AND WAS LEVEL AT 6000 FT BEING HANDLED BY NEW YORK DEP CTL. I WAS PNF. BOTH I AND THE CAPT THOUGHT WE HEARD OUR CTLR CLR US TO 16000 FT. I ACKNOWLEDGED THE CLRNC WITH A FULL READBACK AND THE CAPT CONFIRMED THE ALT AND BEGAN A CLB. AS WE PASSED THROUGH ABOUT 6500 FT; OUR CTLR TOLD US TO MAINTAIN 6000 FT. THE CAPT IMMEDIATELY STOPPED OUR CLB AND RETURNED TO 6000 FT. I ASKED THE CTLR IF WE WERE NOT CLRED TO 16000 FT AND HE SAID NO; IT WAS FOR ANOTHER ACFT. OUR FLT NUMBER WAS AGG AND THERE WERE AT LEAST 3 OTHER FLTS ON THE SAME FREQ WHOSE NUMBER ENDED IN G; OR GG. ACR BJOGG; ACR AOGG; AND ACR BOHG; IF I REMEMBER CORRECTLY. I BELIEVE WE HEARD AND RESPONDED TO ACR B0GG'S CLRNC TO 16000 FT. UNFORTUNATELY; THE CTLR DID NOT CATCH OUR ERROR WHEN I READ BACK THE CLRNC. THIS WAS THE FIRST LEG OF A PAIRING. BOTH I AND THE CAPT WERE WELL RESTED. WE HAD NOT FLOWN TOGETHER BEFORE; BUT OUR RAPPORT AND CRM WERE EXCELLENT. WE BOTH BELIEVE WE HEARD OUR OWN FLT NUMBER AGG 'CLRED TO 16000 FT.' OBVIOUSLY WITHOUT THE TAPES WE CANNOT BE SURE IT WAS OUR ERROR AND NOT THE CTLR'S. I BELIEVE THE MAIN CAUSES OF THIS ERROR ARE: 1) TOO MANY SIMILAR CALL SIGNS ON THE SAME FREQ. 2) CTLR TOO BUSY TO CATCH INCORRECT READBACK. 3) HIGH WORKLOAD FOR BOTH AIR CREW AND CTLR IN CONGESTED AIRSPACE. CORRECTIVE ACTIONS INDICATE A NEED FOR SLOWER RATES OF SPEECH; MORE VIGILANCE WHEN DEALING WITH SIMILAR CALL SIGNS ON CONGESTED FREQS; AND INCREASED ATTN PAID TO READBACKS BY CTLRS. ALSO STANDARD READING OF FLT NUMBERS. SUPPLEMENTAL INFO FROM ACN 478558: CONTACTED CTLR ON LANDLINE. TFC SEPARATION WAS NOT COMPROMISED. OTHER FACTORS: VERY EARLY WAKE-UP FOR AN EARLY SHOW WITH LESS THAN NORMAL SLEEP; THUS NOT AS ALERT FOR MISTAKES.