B737-500 FLT CREW DSNDS THROUGH THEIR ASSIGNED ALT ON APCH TO MCO.

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WE HAD DIFFICULTY CONFIRMING AN ALT CLRNC; AND DSNDED THROUGH THE ASSIGNED ALT. WE HAD BEEN FLYING THE MINEE 3 ARR TO ORLANDO AND; AS I RECALL; HAD BEEN VECTORED OFF WHILE IN THE VICINITY OF THE LAKELAND VOR. THE ATC CTLR HAD ASSIGNED A DSCNT TO 12000 FT; AND ASKED FOR A 'GOOD RATE DOWN DUE TO TFC.' I READ BACK THE CLRNC TO 12000 FT; ACKNOWLEDGING THE INSTRUCTION FOR THE DSCNT RATE. AS WE DSNDED; THE FO ASKED 'IF THE CLRNC WAS TO ONE-TWO OR ONE-THREE THOUSAND.' I REPLIED THAT I THOUGHT I'D HEARD 12000 FT; BUT WOULD CONFIRM THE CLRNC. ADDITIONAL FACTORS WERE THE PUBLISHED 13000 FT RESTR AT THE ANDRO FIX (WE WERE OFF THE ARR; BUT THOUGHT THAT MIGHT STILL BE A CTLR'S CONSIDERATION); AS WELL AS A TCAS TARGET AHEAD THAT APPEARED TO BE CONVERGING. I THOUGHT THAT TARGET MAY HAVE BEEN THE REASON FOR THE 'GOOD RATE' INSTRUCTION. IT INDICATED A LOWER ALT -- AS I RECALL; IT WAS ROUGHLY 1000 FT BELOW US INITIALLY. AS WE DSNDED; WE WERE UNABLE TO ASK FOR CONFIRMATION AS THE FREQ WAS CONGESTED. THE CTLR WAS EXTREMELY BUSY; AND EACH TIME I TRIED TO KEY THE MIKE; ANOTHER CALL INTERRUPTED. AS WE APCHED 13000 FT; WE WANTED VERY MUCH TO CONFIRM THE ASSIGNMENT; BUT THE RADIO WAS STILL CONGESTED. NORMALLY; ONE WOULD PAUSE BRIEFLY BEFORE CONTINUING THE DSCNT; BUT WE WERE STILL CONCERNED WITH: 1) THE POSSIBILITY THAT 12000 FT WAS CORRECT; AND THAT I HAD READ IT BACK WITHOUT CORRECTION BY THE CTLR; 2) THE 'GOOD RATE DUE TO TFC' INSTRUCTION; 3) THE TCAS TARGET WAS STILL AHEAD; AND NOW CO-ALT. WE CONTINUED THE DSCNT AND PASSING ROUGHLY 12600 FT THE CTLR ASKED THAT WE VERIFY WE WERE MAINTAINING 13000 FT. I REPLIED 'NEGATIVE; DSNDING TO 12000 FT.' HE ASKED THAT WE MAINTAIN 13000 FT; WE REPLIED THAT WE WOULD; AND CLBED BACK UP. THERE WAS NO POSSIBILITY OF FURTHER DISCUSSION; AS THE FREQ REMAINED QUITE BUSY; AND WE WERE HANDED OFF SHORTLY THEREAFTER. THE DIFFICULTY IN THIS CASE WAS NOT A NEW ONE -- VOICE COMS WORKS WELL; BUT A HVY WORKLOAD MAKES IT DIFFICULT. I THOUGHT I HEARD ONE ALT ASSIGNMENT; AND READ IT BACK. I BELIEVE THE FO AND I WERE BOTH FOCUSED ON THE JOB AT HAND; AND LISTENING ATTENTIVELY; BUT APPARENTLY THE ERROR IN THIS CASE WAS MINE; SINCE THE CTLR MUST HAVE SAID '13000 FT' WHEN HE ASSIGNED OUR ALT. HE DID NOT NOTICE MY ERROR WHEN I READ IT BACK; I BELIEVE DUE TO THE HVY WORKLOAD. WE ALL WORK TO MAKE OUR AIR TFC SYS SUCCESSFUL. I BELIEVE THE TWO OF US IN THE COCKPIT WERE PAYING ATTN. DESPITE THAT; I STILL MADE AN ERROR. FREQ CONGESTION THWARTED MY ATTEMPTS TO CONFIRM THE CLRNC ONCE A DOUBT SURFACED. I DON'T KNOW IF THERE IS A BETTER WAY TO MANAGE INBOUND TFC (TO PREVENT CTLR OVERLOAD); PERHAPS IN FUTURE YRS; A DATA LINK SYS COULD REDUCE THE POSSIBILITY OF THESE EVENTS.