FLT CREW CONFUSED BY CTLR'S USE OF ACFT CALL SIGN DIGITS AND RWY NUMBERS.

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WE; ACR X BLANKETY; WERE #1 FOR TKOF ON RWY 18R AT MEM. ACR X BLANKETY 1 WAS ALSO ON THE FREQ; BUT I'M NOT SURE OF HIS POS. MEM TWR STATED; 'ACR X BLANKETY-1-8-RIGHT-POS-AND HOLD;' (NO PAUSES). BEFORE MY F/O COULD GET TO HIS MIC TO RESPOND; WE HEARD; 'ACR X BLANKETY 1; POS AND HOLD.' F/O QUERIED TWR IF THE CALL WAS FOR ACR X BLANKETY; WHEREUPON TWR REPEATED HIS INSTRUCTION EXACTLY AS BEFORE. AGAIN WE HEARD THE REPLY FROM OUR COLLEAGUES; 'ACR X BLANKETY 1; POS AND HOLD.' F/O TRIED AGAIN TO CLARIFY THE ISSUE; THIS TIME ASKING; 'ARE YOU TALKING TO ACR X BLANKETY 1 OR ACR X BLANKETY?' THIS TIME TWR INSTRUCTED; 'ACR X BLANK ZERO; 1-8-RIGHT-POS AND HOLD.' WE REPLIED AND TAXIED AS INSTRUCTED. DURING A MOMENTARY LULL A FEW MOMENTS LATER; I SUGGESTED; 'PERHAPS IT WOULD BE CLEARER IF YOU SEPARATED THE CALL SIGN AND THE RWY.' TWR RESPONDED WITH A; 'ROGER;' BUT HE CONTINUED AS BEFORE. SORRY THE PRINTER DIDN'T SPACE WELL; HOPE YOU CAN MAKE IT OUT. IT'S OBVIOUSLY BETTER THAN MY HAND. MY QUESTION: HOW DO I MAKE THE SUGGESTION IN WRITING? THERE IS NO MECHANISM OF WHICH I AM AWARE OF WHEREBY I CAN SUGGEST THIS TO THE FAA.