Narrative:

Under approach control, read back clearance to descend from 10000 to 7000'. The first officer was working radios and I acknowledged the descent to 7000' and began it. At 7600' the controller said our altitude should be 10000'. We told him we read back 7000' and he said that was for another air carrier, not us. He then cleared us to continue to 7000'. There was a lot first officer traffic on the frequency. The controller was slurring the airline; i.e., it was hard to understand air carrier a vs air carrier B, so we caught the 1 in the transmission and read it back. We should not have assumed the controller was listening close enough to hear our readback. If he did hear it, he may also have confused it with air carrier B. I was surprised though when he let us descend so far below our cleared altitude before alerting us.

Related reports:

Google
 

Original NASA ASRS Text

Title: FLT CREW OF ACR MLG ARRIVING PHX APPARENTLY TAKES A DESCENT CLRNC INTENDED FOR ANOTHER FLT.

Narrative: UNDER APCH CTL, READ BACK CLRNC TO DSND FROM 10000 TO 7000'. THE F/O WAS WORKING RADIOS AND I ACKNOWLEDGED THE DSNT TO 7000' AND BEGAN IT. AT 7600' THE CTLR SAID OUR ALT SHOULD BE 10000'. WE TOLD HIM WE READ BACK 7000' AND HE SAID THAT WAS FOR ANOTHER ACR, NOT US. HE THEN CLRED US TO CONTINUE TO 7000'. THERE WAS A LOT FO TFC ON THE FREQ. THE CTLR WAS SLURRING THE AIRLINE; I.E., IT WAS HARD TO UNDERSTAND ACR A VS ACR B, SO WE CAUGHT THE 1 IN THE XMISSION AND READ IT BACK. WE SHOULD NOT HAVE ASSUMED THE CTLR WAS LISTENING CLOSE ENOUGH TO HEAR OUR READBACK. IF HE DID HEAR IT, HE MAY ALSO HAVE CONFUSED IT WITH ACR B. I WAS SURPRISED THOUGH WHEN HE LET US DSND SO FAR BELOW OUR CLRED ALT BEFORE ALERTING US.

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.