ACR MLG RECEIVED AMENDED CLRNC ALT ASSIGNMENT AFTER CROSSING THE CLRNC ALT.

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I WAS FLYING RNO-LAX. SHORTLY AFTER HDOF FROM RNO DEP TO ZOA IT WAS OBVIOUS THAT CTLR TRNING WAS IN PROGRESS. FEMALE VOICE (ATC) WAS SHAKY AND UNCERTAIN SOUNDING. OCCASIONAL MALE ATC VOICE CAME ON FREQ TO CORRECT MISTAKES. CENTER CLRED US TO FL230. SHORTLY THEREAFTER; FEMALE VOICE TELLS US TO 'MAINTAIN 14000'!' SHE SOUNDED CONFUSED AND HURRIED. WE WERE CLBING THROUGH 14400' AT THAT TIME ON THE WAY TO FL230. I WAS ABLE TO STOP THE CLB AT 14200' AND ENGAGED ALT HOLD MODE OF AUTOPLT. I HAD F/O IMMEDIATELY CONFIRM OUR NEW ALT ASSIGNMENT AND CONFIRM THAT SHE NOW WANTED US TO DSND BACK TO 14000' MSL. ATC DID NOT ANSWER THAT CALL OR ANY OF OUR NEXT 3 CALLS OVER A SPAN OF ABOUT 45 SECS. (IT SEEMED LIKE A YR!). WE HAD NEVER ACKNOWLEDGED THAT WE WOULD OR COULD MAINTAIN 14000'; SO HER SILENCE WAS EVEN MORE PERPLEXING. I FINALLY GRABBED THE MIC AND SAID FIRMLY; 'CENTER; WHAT DO YOU WANT XX-FLT XXXX TO DO?' NOT KNOWING WHERE OUR TFC WAS; AT 15000'; 14500 OR WHATEVER; I DIDN'T WANT TO LEAVE 14700' UNTIL I GOT A RESPONSE. SHE FINALLY SAID; 'MAINTAIN FL230.'