A B737-300 CREW; DSNDING INTO HOU; MISINTERPED A CLRNC AND STARTED FINAL DSCNT PREMATURELY. NUMEROUS CALLS FROM ATC WENT UNDETECTED DUE TO A #1 VHF PROB.

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ALL ASPECTS OF THE FLT APPEARED NORMAL UNTIL 4 MI FINAL WHEN; NOT HAVING HEARD ANY RADIO XMISSION FOR SEVERAL MI; I ASKED APCH CTL IF I SHOULD CONTACT TWR. I THOUGHT IT ODD THAT THE CTLR SAID 'I GUESS THE OTHER GUY IS THROUGH TALKING TO YOU; CONTACT TWR.' TAXIING IN; I WAS GIVEN A PHONE NUMBER TO CALL AT HOUSTON APCH. MY FO AND I REVIEWED THE FLT AND COULD NOT THINK OF ANY ABNORMALITIES; SO I WAS A BIT CURIOUS. I BELIEVE I TALKED TO THE PERSON WHO WAS ACTUALLY CTLING US AND WAS SURPRISED WHEN HIS FIRST COMMENT WAS TO ASK IF WE WERE OK. I WAS STUNNED WHEN HE TOLD ME WE HAD NEVER BEEN CLRED TO LEAVE 3000 FT ON THE APCH. APPARENTLY HE HAD BEEN TRYING TO TALK TO US FOR SEVERAL MI; BUT WE HAD HEARD NOTHING. THE FO WAS FLYING; DSNDING TO OUR CLRED ALT OF 3000 FT ABOUT 17 MI OUT; WHEN ANOTHER COMPANY FLT CHKED IN AT 6000 FT WITH THE FIELD IN SIGHT. THINKING WE HAD CALLED; THE CTLR CLRED US FOR A VISUAL APCH. WHEN I TOLD HIM IT WAS NOT US AND THAT WE DID NOT HAVE THE FIELD IN SIGHT; HE TOLD US TO JUST INTERCEPT THE LOC. JUST AFTER PASSING OVER ELLINGTON WHILE TURNING TO INTERCEPT WE WERE GIVEN A TA ABOUT AN ACFT TURNING FINAL IN FRONT OF US. AT 3000 FT; ON THE LOC; WE BEGAN TO CONFIGURE AS THE GS CAME ALIVE. I NOTED THAT THE APCH MODE HAD NOT YET BEEN SELECTED; LOOKED AWAY FOR THE TFC THEN CAME BACK INSIDE AND SAW THAT THE APCH MODE WAS ACTIVATED. THE APCH WAS COMPLETED TO A NORMAL LNDG. DURING THE FLT FROM MSY; BOTH THE FO AND I NOTICED A CLICKING SOUND IN THE #1 RADIO WHEN XMITTING -- ALMOST LIKE THE XMITTER WAS CUTTING OUT. NO RECEIVING AGENCY HAD SAID ANYTHING ABOUT A PROB; BUT I HAD ASKED MAINT TO REPLACE THE CTL HEAD AS A PRECAUTION. I DO NOT THINK OUR RECEIVER HAD FAILED; BUT WE DEFINITELY HEARD NOTHING BTWN THE TA AND THE RESPONSE TO MY REQUEST TO CONTACT TWR. THE FO ACTIVATED HIS APCH CLRNC CUE WHEN WE WERE ORIGINALLY CLRED FOR THE VISUAL APCH; BUT DID NOT CHANGE IT BACK WHEN THAT CLRNC WAS CHANGED. MY CUE (APCH MODE SELECTED) WAS ACTIVATED WHEN I XCHKED IT AS WE APCHED THE GLIDE PATH. NEITHER OF US REALIZED WE HAD NOT BEEN CLRED OUT OF 3000 FT FOR THE APCH. I HAD A VERY GOOD; CONGENIAL CONVERSATION WITH THE APCH CTLR; WHO CONCLUDED 'NO HARM; NO FOUL.' DEFINITELY A LAPSE IN CONCENTRATION ON A 'NORMAL; ORDINARY' APCH. SUPPLEMENTAL INFO FROM ACN 539104: WE WERE VECTORED IN HIGH AND CLOSE TO THE FIELD TO INTERCEPT FINAL UNDER VMC. WHILE DSNDING; PAST 5000 FT FOR 3000 FT; APCH CTL ISSUED A CLRNC TO ANOTHER ACFT USING OUR CALL SIGN 'COMPANY FLT NUMBER.' THE CAPT CORRECTED APCH CTL. OUR VHF #1 RADIO WAS 'TICKING' ON THIS FIRST LEG AND WE HAD CALLED MAINT TO COME CHK IT UPON LNDG. AFTER INTERCEPTING FINAL; THE ACFT WAS CONFIGURED NORMALLY. WHAT WE DID NOT KNOW WAS THAT APCH INTENDED TO LEAVE US AT 3000 FT UNTIL WE PASSED W OF ELLINGTON FIELD (6 DME ON FINAL). VHF #1 WAS QUIET AS WE APCHED GS INTERCEPT AT 3000 FT; APPROX 9 DME. I INCORRECTLY SELECTED APCH MODE ON THE AUTOPLT MCP AND BEGAN TO DSND ON THE GS. ATC CALLED TWICE TO ASK US TO HALT OUR DSCNT. SINCE OUR VHF #1 WAS BAD; WE DID NOT HEAR THESE CALLS. I INCORRECTLY INTERPED ATC'S INITIAL CLRNC -- THE ONE USING OUR CALL SIGN; BUT INTENDED FOR THE OTHER ACFT -- AS A CLRNC TO DSND FOR A VISUAL APCH.