AN A319 FLT CREW AND A ZJX CTLR GET CONFUSED AS TO WHAT ALT THE ACFT WAS CLRED TO DSND TO.

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WHILE FLYING DIRECT TO OMN; WE RECEIVED A CLRNC THAT WE UNDERSTOOD TO BE 'CLRED TO CROSS 90 MI N OF OMN AT FL210.' THE FO READ BACK 'ROGER; CLRED TO CROSS 90 MI N OF OMN AT FL210.' I SET FL210 IN THE ALT WINDOW; AND THE FO VERIFIED FL210. WE BEGAN THE DSCNT TO MAKE THE XING RESTR. JUST PRIOR TO LEVELING OFF; THE CTLR ASKED US TO VERIFY ALT; AND THE FO SAID 'LEVELING AT FL210.' THE CTLR SAID 'FL210? -- YOU WERE CLRED TO FL310.' THE FO RESPONDED; 'SIR; WE UNDERSTOOD FL210 AND READ BACK FL210; DO YOU NEED US TO CLB?' THE CTLR; AFTER A PAUSE; SAID 'NO; MAINTAIN FL210.' THERE WERE NO TFC CONFLICTS AND NO TCASII ADVISORIES. WE ALSO HAD A JUMPSEAT RIDER; ANOTHER CAPT; AND HE LATER TOLD US HE THOUGHT HE HEARD THE CLRNC TO BE FL210. JUST AS SOON AS WE ACKNOWLEDGED THE ORIGINAL CLRNC TO FL210; ANOTHER ACFT MADE AN IMMEDIATE RADIO CALL; WHICH COULD HAVE DISTR THE CTLR'S ATTN FROM ACKNOWLEDGING OUR READBACK. WE WERE ALL FORTUNATE THERE WAS NO CONFLICT DURING THIS INCIDENT. POSSIBLE CORRECTIVE MEASURES COULD BE MORE ATTENTIVENESS FROM BOTH ENDS. IF SOMETHING DOESN'T SEEM EXACTLY RIGHT; IT SHOULD BE QUESTIONED. TO ME; IT SEEMED A LITTLE EARLY TO DSND THAT EARLY TO THAT ALT. EVEN THOUGH OUR ACKNOWLEDGEMENT WAS NOT QUESTIONED BY THE CTLR; IF I HAD QUESTIONED THE CLRNC; IT MAYBE COULD HAVE BEEN AVOIDED.