AN MD80 FLT CREW DEVIATED FROM THE HEADING TRACK REQUIREMENTS OF THE MCCARRAN TWO DEP PROC 3 NM W OF LAS; NV.

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LOCATION -- W OF RWY 25R LAS DEP. MCCARRAN 2 DEP REQUIRES HEADING 190 DEGS AT 3 DME OF LAS. AFTER AIRBORNE; DURING INITIAL FREQ CHANGE FROM TWR TO THE DEP CTL; BOTH FO AND I THOUGHT TO OVERHEAR 'MAINTAIN RWY HEADING' WITH THE CALL SIGN BLOCKED. THE DEP FREQ WAS VERY CONGESTED. WE ELECTED TO STAY ON RWY HEADING WHILE I TRIED TO CONTACT THE CTLR TO VERIFY THE INSTRUCTION. THE FREQ WAS VERY CONGESTED. BY THE TIME I WAS ABLE TO ACTUALLY TALK TO THE CTLR TO VERIFY THE RWY HEADING WAS FOR US; WE EXCEEDED THE 3 DME LIMIT. WE WERE SUPPOSED TO ADHERE TO THE SID; AND TURN AT 3 DME. 'MAINTAIN RWY HEADING' INSTRUCTION WAS NOT FOR US. I CONTACTED THE DEP CTL SUPVR LATER AND DISCUSSED THE SIT; AND WAS TOLD THAT THERE WERE NO TFC CONFLICT OR ANY OTHER OPERATIONAL CONFLICT WERE PRESENT AT THAT TIME. SUPPLEMENTAL IN FROM ACN 595085: FREQ/ALT: 125.9/5000 FT. MCCARREN 2 SID. WE WERE UNABLE TO ESTABLISH RADIO CONTACT WITH DEP CTL DUE TO FREQ CONGESTION; BUT THOUGHT I HEARD THE CTLR SAY MAINTAIN RWY HEADING. THE WX WAS VFR CLR SKIES; WITH UNLIMITED VISIBILITY.