A B757 CARGO FLT DSNDS PAST THEIR CLRED TO ALT WHEN THE CREW MISUNDERSTANDS THE CLRNC 80 MI E OF OAL; NV.

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THE B757 WAS LEVEL AT FL350 HDG APPROX 260 DEGS DIRECT TO OAL VOR (80 NM E) FOR THE EL NIDO 4 ARR INTO SAN JOSE; CA. ZLC STATED 'ACR XYZ; DSND AND MAINTAIN FL310.' THE CLRNC WAS READ BACK AS 'OUT OF FL350 FOR FL310; ACR XYZ.' FL310 WAS SET INTO THE ALT ALERT WINDOW BY THE FO AND ACKNOWLEDGED BY THE CAPT. FL310 WAS THEN ENTERED INTO THE FMC TO INITIATE A 1000 FPM CRUISE DSCNT PROFILE. WHILE PASSING APPROX FL325; ZLC ASKED 'ACR XYZ; ARE YOU LEVELING AT FL330?' THE CREW RESPONSE WAS 'NEGATIVE; OUR LAST CLRNC WAS FOR FL310; ACR XYZ.' ZLC; 'ACR XYZ; MAINTAIN FL330; EXPEDITE CLB; ADVISE WHEN REACHING.' CREW; 'ROGER; WE'RE RAPIDLY CLBING TO FL330; WE'LL ADVISE.' THE CAPT QUICKLY TRANSITIONED FROM A 1000 FPM DSCNT TO A 2500 FPM CLB AT APPROX FL321. FL330 WAS QUICKLY ESTABLISHED AND ZLC WAS ADVISED. THE CTLR RESPONDED BY SAYING THAT HE MAY HAVE MISSED OUR READBACK. A SHORT TIME LATER; HE INSTRUCTED US TO CONTACT ZOA. THE CTLR'S TONE AND DEMEANOR WAS CALM AND CONCILIATORY; SEEMING TO INDICATE THAT NO TFC CONFLICT OCCURRED. THIS EVENT HIGHLIGHTS 2 POTENTIAL HAZARDS THAT NEGATIVELY IMPACT ATC COMS: 1) CTLRS WORKING MULTIPLE FREQS AND PERFORMING MULTIPLE TASKS (IE; GND CTL; TWR; CLRNC DELIVERY; APCH CTL). IN THIS CASE IT APPEARED THAT THE CTLR MAY HAVE BEEN WORKING MORE THAN 1 FREQ. THIS CAN LEAD TO CONFUSION WHEN 2 ACFT ON SEPARATE FREQS XMIT TOGETHER TO THE SAME CTLR. 2) CURRENT REGS DO NOT HOLD CTLRS ACCOUNTABLE IF A CLRNC IS READ BACK INCORRECTLY. THIS LACK OF MUTUAL RESPONSIBILITY REDUCES THE EFFECTIVENESS OF CLRNC READBACKS. SUPPLEMENTAL INFO FROM ACN 474224: AT THE TIME THERE WAS MODERATE ATC TFC. ALL STANDARD ATC AND COMPANY PROCS WERE FOLLOWED. OUR ACFT WAS NON TCASII.