FA20 CREW RPTED THE CTLR ISSUED ACFT ALTS HE LATER QUESTIONED.

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WE HAD ALREADY BEGUN THE TURN AND DSCNT. WHILE APCHING FL270 IN THE DSCNT; ATC TOLD US TO STOP OUR DSCNT AT FL270 AND TO SLOW TO 250 KTS AT THIS TIME. BY THE TONE IN ATC'S VOICE; I COULD TELL SOMETHING WASN'T EXACTLY R. I QUESTIONED ATC TO VERIFY THAT THE PREVIOUS INSTRUCTION WAS TO DSND TO FL250. ATC STATED THAT WAS NOT THE CASE AND THAT HE WANTED US TO SLOW TO 250 KTS; BUT NOW WE COULD STAY AT FL270 AND THAT WAS OK. I IMMEDIATELY ASKED THE FO WHAT EXACTLY WAS SAID BTWN HIM AND ATC. THE FO SAID HE WAS POSITIVE THAT THE CTLR TOLD HIM TO DSND TO FL250 AND THAT HE READBACK THAT WE WOULD DSND TO FL250. FURTHERMORE; THE FO STATED THAT THE CTLR HAD MADE A SIMILAR MISTAKE WITH ANOTHER ACFT WHILE I WAS OFF FREQ. THE BIGGEST ISSUE FOR MYSELF IS THAT I WAS NOT MONITORING CTR WHILE I WAS TALKING TO FLT SERVICE. AS A RESULT; I DON'T KNOW EXACTLY WHAT WAS SAID BTWN CTR AND THE FO. I SHOULD HAVE MONITORED CTR AT THE SAME TIME I WAS TALKING TO FLT SERVICE. THIS WAY I COULD HAVE POSSIBLY CAUGHT A MISTAKE IF ONE ACTUALLY OCCURRED.