B737. ALT CLRNC IN DISPUTE IN PHX CLASS B AIRSPACE.

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JUST PAST ARLIN AT 11000 FT; 250 KTS; RECEIVED CLRNC TO 075 DEG HDG AND (WE THOUGHT) 7000 FT; AND FO READ BACK 7000 FT; WITHOUT COMMENT FROM ATC. FO WENT OFF FREQ TO TALK TO COMMANDING OFFICER; AND I RECEIVED A FREQ CHANGE SHORTLY THEREAFTER. UPON CHKING IN ON NEW FREQ; I WAS TOLD I NEEDED TO CLB TO 9000 FT; THAT WE WERE CLRED TO 10000 FT; NOT 7000 FT. (ALT WAS APPROX 8400 FT AT THAT TIME.) PROMPTLY COMPLIED. NO TFC CONFLICT EXISTED AND NO FURTHER COMMENTS RECEIVED FROM ATC. I WAS PRETTY SURE WE BOTH HEARD 7000 FT; BECAUSE THE FO INITIALLY READ BACK 070 DEGS INSTEAD OF 075 DEGS ON THE HDG; AND I THOUGHT TO MYSELF THAT 'HE GOT THE 7'S MIXED UP.' CONTRIBUTING FACTORS: FLYING A NON FMS ACFT (WE DON'T FLY IT OFTEN AT MY BASE) WHICH MEANT A LITTLE MORE DISTR ON MY PART ADDING UP NUMBERS FOR XING RESTRS. ALSO; THIS WAS THE FO'S VERY FIRST MONTH ON LINE AFTER FINISHING IOE. SUPPLEMENTAL INFO FROM ACN 477116: WHILE DSNDING ON ARLIN 2 TO PHX; JUST E OF ARLIN WE RECEIVED INSTRUCTIONS TO TURN L 075 DEGS AND DSND TO 7000 FT. I READ BACK THE CLRNC AND THE CTLR DID NOT CORRECT ME. AS THE CAPT STARTED THE DSCNT; I WENT OFF FREQ TO CHK IN WITH COMPANY. AT ABOUT 8400 FT; THE CAPT STARTED TO CLB AND SAID THE CTLR HAD TOLD US WE WERE ONLY CLRED TO 10000 FT AND WANTED US BACK UP TO 9000 FT. WE COMPLIED WITH THE INSTRUCTIONS. THERE WERE NO TFC CONFLICTS APPARENT AND WE CONTINUED TO PHX WITH NO MORE MENTION OF THE SIT FROM ATC.