A B767 CREW; ARRIVING PHL; MISUNDERSTOOD AN 'EXPECT TO CROSS' CLRNC AS BEING A CLRNC TO CROSS; STARTING DSCNT PREMATURELY.

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DURING THE INITIAL DSCNT INTO PHL; ATC ADVISED US THAT WE COULD EXPECT TO CROSS THE TERRI INTXN ON THE DUPONT 4 ARR AT 10000 FT AND 250 KTS. THIS WAS AN EXPECT ONLY ADVISORY -- WE WERE TO MAINTAIN OUR PRESENT ALT OF 15000 FT. I READ BACK THE ADVISORY AS AN 'EXPECT' ONLY CLRNC. THE CAPT WAS THE PF FOR THIS LEG. AFTER READING BACK THE EXPECT CLRNC; I ADVISED THE CAPT THAT I WOULD STOP MONITORING COM #1 RADIO (IN USE FOR ATC COM) SO THAT I COULD MAKE A FAREWELL ANNOUNCEMENT TO OUR PAX. HE ACKNOWLEDGED ME; AND I PROCEEDED TO MAKE THE ANNOUNCEMENT. WHEN I RETURNED TO COM #1 RADIO (RESELECTED #1 RADIO); MY ATTN WAS CAUGHT BY THE UNWINDING ALTIMETER. I ASKED THE CAPT IF ATC HAD CLRED US TO DSND TO 10000 FT. HE SAID YES. SHORTLY THEREAFTER; ATC ADVISED US THAT WE HAD NOT BEEN GIVEN A DSCNT CLRNC AND HE RESTATED THAT IT HAD BEEN AN 'EXPECT TO CROSS' ONLY. I ASKED THE CAPT AGAIN IF WE HAD INDEED BEEN CLRED TO DSND DURING THE TIME I HAD BEEN TALKING TO THE PAX. HIS REPLY WAS 'I THOUGHT WE WERE.' THE CTLR ALSO STATED THAT OUR ALTDEV DID NOT CREATE ANY CONFLICTS. OUR AIRLINE HAS AN ALT AWARENESS PROGRAM THAT HAS WORKED VERY WELL. IT HAS BEEN ADOPTED BY SEVERAL OTHER AIRLINES. IT DOES NOT; HOWEVER; ADDRESS 'EXPECT TO CROSS' CLRNCS. IT IS USED TO VERIFY ALT SELECTION IN THE ALT WINDOW OF OUR MCP OF THE APFDS. I WILL SUGGEST TO OUR SAFETY DEPT THAT IT BE EXTENDED TO INCLUDE THAT BOTH PLTS VERBALIZE THE 'EXPECT TO CROSS' CLRNCS AS SUCH. THE MAIN CAUSE FOR OUR ALTDEV WAS MY ASSUMPTION THAT THE CAPT UNDERSTOOD THE CLRNC SIMPLY BECAUSE HE WAS ALSO LISTENING TO THE RADIO XMISSIONS.