ACR FLT CREW HEARD ALT ASSIGNMENT AS 12000 AND READBACK THAT ALT. DEP CTLR WANTED ACFT TO CLIMB TO 10000; BUT FAILED TO NOTE INCORRECT READBACK OF ALT. OPERATIONAL DEVIATION.

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WE HAD JUST DEPARTED PHL ARPT. THE DEP CTLR ON INITIAL CONTACT ADVISED US 'AFTER 2000' ALT TURN LEFT TO 180 DEG HEADING CLIMB AND MAINTAIN 12000'. WE (COPLT) READ BACK HIS CLRNC VERBATIM. I DIALED IN 12000' ON THE ALT WARNING DEVICE. CLIMBING THROUGH 11000' PHL DEP CALLED US AND ADVISED US HE HAD ONLY GIVEN US 10000'. I IMMEDIATELY PLACED POWER TO IDLE AND DESCENDED TO 10000'. AT WHICH TIME PHL DEP ADVISED US TO CONTACT NEW YORK CENTER. I TOLD MY COPLT TO CONFIRM WITH NEW YORK CENTER WHAT ALT WE WERE TO MAINTAIN; SINCE PHL DEP HAD BEEN TOO BUSY TO ASK A SECOND TIME TO VERIFY 10000' BEFORE HE SWITCHED US TO NEW YORK CENTER. NEW YORK CENTER CONFIRMED 'MAINTAIN 10000'.' BOTH MY COPLT AND I HEARD PHL GIVE US 12000' AS OUR INITIAL ALT AND 12000' IS WHAT WE READBACK TO DEP CTL. DEP CTL DID NOT CORRECT THIS READ BACK. UPON ARRIVING AT OUR DEST; I CALLED PHL DEP ON THE TELEPHONE. THE SUPERVISOR WAS AWARE OF THE SITUATION AND APOLOGIZED FOR THE CTLR NOT CORRECTING OUR ALT READBACK. SHE SAID SHE HAD NOT YET LISTENED TO THE TAPE BUT THE CTLR SAID HE INITIALLY GAVE US 10000. A DEFINITE COMMUNICATION BREAKDOWN OCCURRED BECAUSE THE COPLT AND I BOTH HEARD 12000 WHEN 10000 MAY HAVE BEEN GIVEN AND THE CTLR NOT CORRECTING US WHEN WE READ BACK 12000. PHL SAID NO CONFLICT HAD OCCURRED AND NO VIOLATION WOULD BE FILED. SUPPLEMENTAL INFORMATION FROM ACN #88454: BOTH THE CAPT AND I INDEPENDENTLY HEARD A CLRNC TO 12000' AS EVIDENCED BY OUR SIMULTANEOUS ACTIONS. I SUPPOSE IT'S POSSIBLE THAT WE BOTH MISTAKENLY HEARD THE SAME INCORRECT ALT ASSIGNMENT. TWO THINGS CONCERN ME REGARDING THE ATC HANDLING WE RECEIVED. ONE; THAT IF IN FACT WE HAD BEEN CLRED TO 10000' WHY HAD I NOT BEEN CORRECTED BY THE CTLR WHEN I ACKNOWLEDGED 12000' AND TWO; WHY WERE WE NOT ADVISED OF OUR ALT DEVIATION UNTIL WE WERE ALMOST 2000 ABOVE WHAT THE CTLR MAINTAINED WE HAD BEEN CLRED TO.