ARTCC CTLR ISSUES CLRNC MEANT FOR ANOTHER ACFT WITH SAME LAST 2 NUMBER CALL SIGN AND DID NOT CATCH MISTAKE WHEN THE RPTR OF A B777 ACKNOWLEDGED WITH HIS FULL CALL SIGN.

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GATWICK (EGKK) INTL ARPT FOR ATL. AT THE TIME OF INCIDENT WE WERE ON FREQ 135.17 (ZOB). I WAS THE PNF; THE CAPT WAS PF WITH AUTOPLT ENGAGED. WE WERE LEVEL AT OUR CRUISING ALT OF FL350. CTR TOLD US TO DSND TO FL330 AND TO EXPEDITE THIS DSCNT. I ACKNOWLEDGED THIS CLRNC AND THE CAPT IMMEDIATELY SELECTED VERT SPD AS THE PITCH MODE ON THE MCP. THE ACFT BEGAN A 1500-2000 FPM RATE OF DSCNT; WHEREUPON THE CTLR THEN MODIFIED THE CLRNC TO BE LEVEL AT FL330 IN 2 1/2 MINS OR LESS. I ALSO READ BACK THIS CLRNC. DSNDING THROUGH FL340; THE CTLR ASKED US TO TURN TO A HDG OF 220 DEGS; WHICH I READ BACK WITH THE ADDITIONAL READBACK THAT WE WERE PASSING FL340 FOR FL330. THE CTLR THEN STATED THAT HE WANTED US TO MAINTAIN FL350. I READ BACK AND CONFIRMED THAT HE WANTED US TO RETURN TO FL350. THE CAPT REVERSED THE DSCNT AND WE RETURNED TO FL350; AS PREVIOUSLY CLRED. I HAVE NO IDEA WHAT CAUSED THIS INCIDENT. THERE WAS NO REASON FOR US TO QUESTION THE CLRNC; BECAUSE OF THE URGENCY IN THE CTLR'S TONE OF VOICE FOR US TO EXPEDITE THIS DSCNT. WE ASSUMED IT WAS FOR TFC SEPARATION; BUT IN RETROSPECT; IT PROBABLY WAS FOR TFC SEPARATION; BUT FOR ANOTHER ACFT. IN FACT; AFTER OUR DSCNT AND CLB BACK TO ALT; WE HEARD WHAT SOUNDED LIKE THE SAME CTLR ISSUE THE SAME CLRNC (DSND TO FL330; BE LEVEL IN 2 1/2 MINS OR LESS) TO ANOTHER ACFT. WE HAD A 2 DIGIT CALL SIGN AND THERE WAS ANOTHER COMPANY ACFT ON THE FREQ WHOSE 4 DIGIT CALL SIGN WAS THE SAME AS OURS. THIS MAY HAVE BEEN A FACTOR. OUR COMPANY IS HEAVILY COMMITTED TO AN ANTI-ALT 'BUST' CAMPAIGN; WHICH I WHOLEHEARTEDLY SUPPORT. IN THIS INCIDENT; THE CAPT AND I BOTH CONFIRMED THAT THIS CLRNC WAS FOR US; FULL READBACKS WERE GIVEN (TO MY KNOWLEDGE); INCLUDING CALL SIGN AND OUR 'DOUBLE-POINT' TECHNIQUE WAS UTILIZED TO CONFIRM THAT THE INTERNAL COMS (COCKPIT) WERE CORRECT AND SET ON THE MCP AND CONFIRMED ON THE PFD. NO TCASII EVENT WAS TRIGGERED IN OUR ACFT.