COMMUTER FLC THOUGHT HEARD TAXI POS AND HOLD; BE READY. TRIED TO CONFIRM CLRNC FOR THEM BUT TAXIED ONTO RWY PRIOR TO HEARING ANY CONFIRMATION.

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SFO WAS DEPARTING 1L/1R AND LNDG 28L/28R. WE REQUESTED DEP FROM 28R INTXN E. TWR INSTRUCTED CROSS 28L HOLD SHORT 28R. WHILE HOLDING SHORT 28R INSTRUCTIONS WERE ISSUED '...POS AND HOLD BE READY FOR IMMEDIATE ACFT ON FINAL 28R.' THE FIRST PART OF THE XMISSION WAS BLOCKED. THINKING IT WAS OURS; I STATED AND INFORMED 'IF THAT WAS MEANT FOR COMPANY; UNDERSTAND POS AND HOLD 28R WE'LL BE READY.' I STARTED IN POS EXPECTING A CONFIRMATION. NO REPLY WAS GIVEN I THOUGHT BECAUSE OF THE CONGESTED FREQ. WE LOOKED TO THE APCH END OF 28R AND DID SEE AN ACFT ON FINAL WHICH APPEARED FAR ENOUGH OUT SO AS NOT TO RAISE ADDITIONAL DOUBTS ABOUT THE INSTRUCTIONS. WE WERE NOW IN POS AND STILL NO REPLY. INSTEAD THE ACFT (LTT) ON FINAL WAS CLRED TO LAND. SEEING US ON THE RWY THE LTT ADVISED THE TWR. THE TWR INSTRUCTED THEM TO GAR; WHICH THEY DID. TWR CONFIRMED OUR POS AND INSTRUCTED US TO CLR THE RWY AND HOLD SHORT AS ORIGINALLY INSTRUCTED; WHICH WE DID. I FEEL THE INCIDENT WAS CONTRIBUTED TO BY FREQ CONGESTION; WHAT I FELT WAS A NEED TO COMPLY WITH THE INSTRUCTIONS IN A TIMELY MANNER; ACTING BEFORE A REPLY TO MY INQUIRY; AND MY PHRASING COULD HAVE BEEN MORE PROFESSIONAL. I SHOULD HAVE STATED 'CONFIRM POS AND HOLD 28R BE READY' AND NOT MOVED UNTIL A REPLY WAS RECEIVED OR SIMPLY 'BLOCKED' AND NOT MOVED WITHOUT NEW INSTRUCTIONS. PRIOR TO THIS INCIDENT I HAVE ALWAYS REQUESTED CONFIRMATION WHENEVER A DOUBT EXISTED AS TO THE RECIPIENT AND/OR CONTENT OF ANY INSTRUCTION. I ASSURE YOU I WILL CONTINUE TO DO SO IN THE FUTURE. THE SCENARIO IN THIS RPT IS AS ACCURATELY AS I CAN RECALL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THIS WAS A VERY BUSY TIME AT SFO. LOTS OF BLOCKED XMISSIONS. FEELS THAT QUERY TO CONFIRM MAY HAVE BEEN BLOCKED AND THUS NO RESPONSE. ANNOYED AT HIMSELF FOR NOT STAYING IN POS UNTIL HE CONFIRMED CLRNC.