ACR FLC RESPOND TO AND ACKNOWLEDGE ALT CHANGE AND MULTIPLE FREQ CHANGES BEFORE BEING NOTIFIED OF A POSSIBLE ZLA OPERROR.

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IT WAS THE CAPT'S LEG AND WE WERE CRUISING AT FL370 ON J9 BTWN LAS VOR AND MLF VOR. WE WERE 30-40 NM N OF LAS VOR TALKING TO LAX ON FREQ 135.55. CTR CLRED US TO FL310 AND MY FO READ BACK OUR CALL SIGN AND THAT WE WERE LEAVING FL370 FOR FL310. I ENTERED A GENTLE DSCNT AT 1000 FPM FOR PAX COMFORT. APCHING FL330 (ABOUT 4 MINS LATER) ZLA GAVE US A FREQ CHANGE TO 125.75 AND WE ACKNOWLEDGED. THIS FREQ WAS IN ERROR AND THE CTLR TOLD US TO RETURN TO OUR PREVIOUS FREQ; SO WE COMPLIED. AT THIS TIME; WHILE APCHING FL324; WE NOTICED AN ACFT ON TCASII PASSING 1400 FT BENEATH US AT FL310 -- OUR CLRNC ALT. WE KNEW SOMETHING WAS WRONG EVEN THOUGH A TCASII TFC ALERT WAS NEVER HEARD. BY THE TIME WE COULD RE-ESTABLISH CONTACT ON FREQ 135.55; WE HAD LEVELED AT FL310 APPROX 50 NM S OF MLF VOR. WE KNEW THAT WE HAD A DIFFERENT CTLR DUE TO A DISTINCTIVELY DIFFERENT VOICE. HE QUERIED US ON OUR ALT AND WE TOLD HIM THAT WE WERE LEVEL AT FL310. HE EXPRESSED SOME CONFUSION AS TO WHY WE WERE AT FL310 AND WE ASSURED HIM THAT IT WAS OUR PREVIOUS CLRNC ALT. HE TOLD US TO MAINTAIN FL310 AND NOT MUCH ELSE WAS SAID BY HIM REGARDING THE INCIDENT. HOWEVER; WE QUICKLY GOT YET ANOTHER CTLR AND HE SOON TOLD US THAT WE HAD BEEN INVOLVED IN A POSSIBLE DEV AND INSTRUCTED US TO PHONE ZLA UPON ARR. LATER INFLT; ZLC TOLD US ON FREQ 133.9 THAT ZLA HAD CALLED TO TELL US THAT THEY WERE ACCEPTING FULL RESPONSIBILITY FOR THE INCIDENT. UPON ARR IN SLC; I SPOKE WITH ZLA. HE REAFFIRMED THAT ZLA HAD ACCEPTED FULL RESPONSIBILITY FOR THE INCIDENT.