B737 CREW DSNDS TOO EARLY AND ENCROACHES ON CLASS D AIRSPACE BELOW THE PHX CLASS B AIRSPACE WHEN CLRNC FOR VISUAL APCH IS CONFUSED.

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BEING VECTORED FOR VISUAL APCH TO PHX RWY 26L BEHIND A XING DASH 8 WHO WAS BEING VECTORED FOR RWY 26R. AFTER CALLING THE ARPT; AND LATER THE DASH 8 IN SIGHT; WE WERE TOLD TO INTERCEPT FINAL FOR RWY 26L BEHIND THE DASH 8. WE HAD PREVIOUSLY BEEN CLRED TO 4000 FT. WE WERE TOLD TO FOLLOW THE DASH 8 (EXCEPT THAT HE WAS GOING TO RWY 27R AND WE WERE GOING OT RWY 26L). WE BOTH THOUGHT WE'D BEEN CLRED FOR THE VISUAL BEHIND THE DASH 8 AND I DIALED 3100 FT INTO THE MCP TO COMPLY WITH THE NORMAL CHARTED VISUAL APCH RESTRS (WHICH ARE FOR NOISE ABATEMENT) EVEN THOUGH WE WERE CLRED FOR THE 'VISUAL' RATHER THAN THE 'CHARTED VISUAL' APCH. AFTER INITIATING THE DSCNT; THE CTLR ASKED US TO RETURN TO 4000 FT TO REMAIN CLR OF THE FALCON (ARPT) TFC PATTERN. PRIOR TO THIS COMMAND; I HAD NOTED TO THE FO THAT GA TFC ON THE L (LOW) WAS IN SIGHT. TELLING US SPECIFICALLY TO FOLLOW THE DASH 8 AND MAINTAIN 4000 FT UNTIL 'X' DME OR UNTIL FURTHER ADVISED WOULD HAVE CLARIFIED THE SIT FOR US. IF IN DOUBT; WE WOULD HAVE QUESTIONED THE CTLR; BUT FOR THE 'VISUAL' WE WERE BOTH CONVINCED WE'D BEEN CLRED.