Narrative:

En route to sjc via el nido VOR, then direct papee intersection. My copilot went off frequency to call FBO and listen to ATIS. We were in a descent from FL260 to FL240. Center gave us clearance from FL240 to FL230. I acknowledged and complied. Just before leveling at FL230, my copilot came back on frequency without advising me, acknowledged a call to descend to 12000 ft, then selected 12000 ft in the altitude selector. The copilot, myself, and another pilot in the jump seat heard a transmission of '12000 ft.' the next transmission was the copilot verifying a TA given by center. We then turned our TCASII to on-screen (mfd) at which point we received a TA. The traffic was at 5 O'clock and 1800 ft below us. Continuing our descent, center then transmitted asking us our altitude. We replied FL210 and descending to 12000 ft. He then told us we were not cleared to descend. We asked to level at FL210. Corrective action: whenever one pilot leaves operating frequency, he needs to state so. When returning, he also needs to state so, then should be briefed by PF as to any changes or condition of flight. In this instance, all 3 pilots heard 12000 ft. Whether it came from the copilot or originated elsewhere, the copilot acknowledged 12000 ft. If uncertainty is felt, always doublechk with controller. Unfortunately, in this instance, all 3 pilots heard 12000 ft. Supplemental information from acn 405555: ATC called us back as we were descending through FL210. We then held FL210 and were then asked to call the center.

Related reports:

Google
 

Original NASA ASRS Text

Title: FLC OF CL65 HAS MISCOM WITH ATC RESULTING IN LTSS DURING DSCNT. TCASII ACTIVATES.

Narrative: ENRTE TO SJC VIA EL NIDO VOR, THEN DIRECT PAPEE INTXN. MY COPLT WENT OFF FREQ TO CALL FBO AND LISTEN TO ATIS. WE WERE IN A DSCNT FROM FL260 TO FL240. CTR GAVE US CLRNC FROM FL240 TO FL230. I ACKNOWLEDGED AND COMPLIED. JUST BEFORE LEVELING AT FL230, MY COPLT CAME BACK ON FREQ WITHOUT ADVISING ME, ACKNOWLEDGED A CALL TO DSND TO 12000 FT, THEN SELECTED 12000 FT IN THE ALT SELECTOR. THE COPLT, MYSELF, AND ANOTHER PLT IN THE JUMP SEAT HEARD A XMISSION OF '12000 FT.' THE NEXT XMISSION WAS THE COPLT VERIFYING A TA GIVEN BY CTR. WE THEN TURNED OUR TCASII TO ON-SCREEN (MFD) AT WHICH POINT WE RECEIVED A TA. THE TFC WAS AT 5 O'CLOCK AND 1800 FT BELOW US. CONTINUING OUR DSCNT, CTR THEN XMITTED ASKING US OUR ALT. WE REPLIED FL210 AND DSNDING TO 12000 FT. HE THEN TOLD US WE WERE NOT CLRED TO DSND. WE ASKED TO LEVEL AT FL210. CORRECTIVE ACTION: WHENEVER ONE PLT LEAVES OPERATING FREQ, HE NEEDS TO STATE SO. WHEN RETURNING, HE ALSO NEEDS TO STATE SO, THEN SHOULD BE BRIEFED BY PF AS TO ANY CHANGES OR CONDITION OF FLT. IN THIS INSTANCE, ALL 3 PLTS HEARD 12000 FT. WHETHER IT CAME FROM THE COPLT OR ORIGINATED ELSEWHERE, THE COPLT ACKNOWLEDGED 12000 FT. IF UNCERTAINTY IS FELT, ALWAYS DOUBLECHK WITH CTLR. UNFORTUNATELY, IN THIS INSTANCE, ALL 3 PLTS HEARD 12000 FT. SUPPLEMENTAL INFO FROM ACN 405555: ATC CALLED US BACK AS WE WERE DSNDING THROUGH FL210. WE THEN HELD FL210 AND WERE THEN ASKED TO CALL THE CTR.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.