ACR USING IMPROPER PHRASEOLOGY OF CALL SIGN CREATES CONFUSION DUE SIMILAR CALL SIGNS.

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I WAS THE CAPT ON FLT ACFT ORD-HDN. ON TAXI OUT ORD METERING 121.67 TOLD US TO SWITCH TO 126.9. I TOLD MY COPLT WHO WAS HANDLING THE RADIO THIS WAS EXTREMELY UNUSUAL. I TOLD HIM TO VERIFY THE FREQ WHICH HE DID AND HE ASKED FOR TAXI INSTRUCTIONS. THE TWR 126.9 TOLD US TO 'GO TO T1.' SO I DID. AS I WAS TAXIING TOWARD T1 I HEARD ANOTHER ACR ON FREQ WHICH WAS FFI AT THAT TIME I REALIZED THAT TWR THOUGHT HE WAS TALKING TO FFI BUT HE WAS TALKING TO US AND WE RESPONDED AS AFFI UP TO THAT POINT. AT THAT TIME MY COPLT SAID HIS CALL SIGN AS A-F-FI AND TOLD OUR LOCATION T1. TWR 126.9 TOLD US ALL TO 'STOP' SO HE COULD SORT IT OUT. HE TOLD US THAT HE WAS JUST ABOUT TO CLR US FOR TKOF (AND WE WERE STILL ON THE INNER TAXIWAY). WE SWITCHED BACK TO GND ON 121.67 AND HENCE GOT A CHEWING OUT. I BELIEVE THE GND CTLR ON 121.67 WAS HANDLING 2 FREQS (THE OTHER BEING 121.75) AND THOUGHT WE WERE FFI CALLING HIM ON 121.75. THUS THE CONFUSION. MY COPLT VERIFIED ALL TAXIING INSTRUCTIONS WITH A FULL READBACK; HOWEVER ORD TWR DID NOT REALIZE THE PROBLEM UNTIL WE USED THE CALL SIGN PROPERLY AS A-F-FI RATHER AFFI.