B777 CREW DID NOT CONTACT APPROPRIATE GND CTL ON TAXI IN.

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WE LANDED ON RWY 19R AND EXITED THE RWY TO TXWY Y WHERE THE TWR SENT US OVER TO GND CTL WHO IMMEDIATELY CLRED US TO HOLD AT TXWY D AND TO CONTACT RAMP. I WAS ON THE L VHF WITH GND CTL. THE FO USED THE R VHF TO MAKE HIS CALL AND THEN SAID TO ME 'RAMP SAYS THE GATE IS OPEN.' WE THEN TAXIED DOWN TXWY D AND INTO GATE XX. JUST AFTER SHUTTING DOWN BOTH ENGS OUR ACR RAMP CTL/OPS CALLED TO TELL US THAT WE SHOULD HAVE CONTACTED DULLES MIDFIELD RAMP CTL FOR TAXI CLRNC FROM TXWY Y INTO THE GATE. FIRST STATEMENT; THE 20-7 PAGES CLRLY EXPLAIN THE PROCS TO BE FOLLOWED FOR BOTH ARR AND DEP AT IAD RELATIVE TO DULLES MIDFIELD RAMP CTL. AT MOST OF ACR'S STATIONS; WHEN THE LCL OPS/RAMP CTL PERSON SAYS THE GATE IS OPEN IT THEN BECOMES THE FLC'S RESPONSE TO TAXI DIRECTLY TO THE GATE UNLESS INSTRUCTED TO DO OTHERWISE. IN THAT BOTH ACR AND DULLES ARPT USE THE TERM 'RAMP' AS A SHORTENED FORM FOR 'DULLES MIDFIELD RAMP CTL' AND 'ACR OPS/RAMP CTL' THE POTENTIAL TRAP SHOULD BE FAIRLY OBVIOUS. AT IAD THE 'RAMP' (DULLES MIDFIELD RAMP CTL) DETERMINES THE TIMING; RTE AND CLRNC TO AND FROM THE GATES. THE OTHER 'RAMP' (ACR AIRLINES OPS/RAMP CTL) SIMPLY VERIFIES THE OPENNESS OF THE GATE AREA. UNFORTUNATELY; IN ACR AIRLINES OPS THE TERMS 'RAMP' OR 'OPS' ARE -- ON A DAILY BASIS -- USED TO CONTACT WHAT MAY BE LOCALLY DESIGNATED AS EITHER 'ACR OPS' OR 'ACR RAMP CTL.' REGARDLESS OF DESIGNATION; EITHER CALL WILL GET A RESPONSE. MOST ALL OF ACR'S STATIONS DO NOT REQUIRE A SUB-SET OF GND CTL (LIKE IAD'S 'DULLES MIDFIELD RAMP CTL') BEFORE GOING DIRECTLY TO ACR'S RAMP CTL FOR 'CLRNC' INTO AN OPEN GATE. BECAUSE OF THE SIMILARITY OF TERMS USED FOR TAXI INSTRUCTIONS AND CLRNCS INTO OPEN GATES; UNLESS ONE IS AWARE OF THE SPECIFIC FREQ BEING USED IT IS SOMEWHAT EASY TO THINK ONE IS TALKING TO 'RAMP' WHEN THEY ARE REALLY TALKING TO 'RAMP.' PERHAPS IF IAD GND CTL ALWAYS STATED THAT ONE SHOULD CONTACT 'MIDFIELD' RAMP CTL; WITH THE APPROPRIATE FREQ; FOR TAXI INSTRUCTIONS INTO THE GATE AREA AND IF ACR'S AIRLINES OPS ALWAYS STATED THAT ONE SHOULD CONTACT 'MIDFIELD' RAMP CTL; WITH THE APPROPRIATE FREQ; FOR TAXI INSTRUCTIONS OUT OF THE GATE AREA THIS POTENTIAL TRAP COULD BE AVOIDED. THE WORDS I HEARD WERE APPROPRIATE AND REASONABLE RELATIVE TO OUR DAILY OPS. UNFORTUNATELY; EVEN THOUGH THEY WERE CORRECT FOR THE PARTY GIVING THEM THEY WERE NOT FROM WHOM I THOUGHT WE WERE TALKING TO AND THEREFORE NOT APPROPRIATE FOR THE FOLLOWING ACTIONS THAT I TOOK. WILL THIS HAPPEN TO ME AGAIN AT IAD -- NOT LIKELY. WILL THIS HAPPEN TO SOMEONE ELSE AT IAD -- VERY LIKELY IF NO CHANGES ARE INSTITUTED.