SF34 EXPERIENCES ALTITUDE EXCURSION AS THE RESULT OF ACCEPTING CLEARANCE FOR ANOTHER FLIGHT WITH A SIMILAR CALL SIGN.

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WE WERE 30 NM N OF IAD AT 4000 FT ON AN ASSIGNED 200 DEG HDG WITH POTOMAC APCH. WE THEN RECEIVED THE CALL 'CONTACT POTOMAC APCH.' WE CHKED IN ON THE NEW FREQ AT 4000 FT AND THE CTLR ACKNOWLEDGED US XXXB. WE REPLIED THAT WE WERE XXXA (WE HAD NOT YET HEARD A CALL FOR AN XXXA AND WERE UNAWARE OF THEIR EXISTENCE TO THIS POINT). A BRIEF EXCHANGE OCCURRED TO STRAIGHTEN OUT OUR CALL SIGN WHICH I BELIEVED WAS DUE TO A TYPO ON OUR STRIP. WE WERE THEN INSTRUCTED TO DSND AND MAINTAIN 3000 FT. UPON REACHING APPROX 3200 FT; THE CTLR ASKED IF WE WERE ABOUT 5 MI N OF IAD; TO WHICH WE REPLIED IN THE NEGATIVE AND THAT WE WERE AT PRESENT 25 MI N. THE CTLR THEN RECOGNIZED WHAT HAD HAPPENED AND WE WERE TOLD THAT WE TOOK OUR COMPANY'S CALL AND TO GO BACK TO OUR PREVIOUS FREQ. UPON SWITCHING BACK; WE HEARD AN XXXB BEING HANDED OFF. WHEN CHKING BACK IN; WE INQUIRED ABOUT OUR ALT; TO WHICH WE WERE TOLD WAS FINE. THE FLT THEN PROCEEDED AS NORMAL UNTIL WE WERE ONCE AGAIN ADDRESSED AS XXXB UPON EXITING RWY 1R BY THE TWR CTLR. (XXXB WAS AT THIS TIME PARKED AT THE GATE.) UPON REACHING THE RAMP; WE DISCOVERED AN XXXC ALSO ON FREQ. AFTER DISCUSSING THE INCIDENT WITH THE CREW FROM XXXB WE BELIEVE THE CTLR DID IN FACT GIVE INSTRUCTIONS THAT XXXA (NOT XXXB) WAS TO CHANGE FREQ BY MISTAKE FOR THE CREW OF XXXB WAS WONDERING WHY THEY THEMSELVES HAD NOT BEEN SWITCHED YET. XXXB ALSO RPTED DIFFICULTY IN COMMUNICATING WITH OPS; AS THEIR CALLS WERE REPEATEDLY READ BACK AS XXXA. THE CTLR NOTICED THAT THE ACFT HE THOUGHT HE TOLD TO DSND WAS NOT DSNDING. WE FOLLOWED INSTRUCTIONS FROM ATC. THE PRACTICE OF ORGANIZING FLT NUMBERS BY BANK MAY BE CONVENIENT FROM A CLERICAL STANDPOINT; BUT IT IS OUTRIGHT DANGEROUS AT THE OPERATIONAL LEVEL.