CE500 FLT CREW RPTS A POSSIBLE ALTDEV RESULTING IN A TCAS RA.

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WE WERE DEPARTING IMC WITH SHOWERS AND LAYERED CLOUDS. I DO NOT RECALL IF WE WERE IMC AT THE TIME; BUT ATC WAS HIGHLY CONGESTED; WITH BLOCKED CALLS AND MISSED CALLS. WE HAD FINISHED ONE OF THE SEVERAL STEP CLBS AND WERE LEVEL AT AN ASSIGNED FLT LEVEL (I BELIEVE FL240); ON AUTOPLT; AND HAD FINISHED SETTING AN INTERIM CRUISE PWR. WE WERE EXPECTING ANOTHER CLB TO OUR REQUESTED ALT. AT SOME POINT THEREAFTER; WE HEARD A CTLR REQUEST US TO DSND TO FL240 AND FOR ANOTHER ACFT TO CLB TO FL270. SINCE WE WERE ALREADY AT THE ASSIGNED FLT LEVEL; THERE WAS CONFUSION IN OUR COCKPIT AND I HESITATED TO DO ANYTHING WITHOUT CLARIFICATION WITH ATC. THE FREQ WAS JAMMED WITH OTHER COMS AND MY COPLT COULD NOT GET THROUGH TO CLARIFY THE COM. SHORTLY THEREAFTER; THE CTLR REPEATED THE REQUEST; ALTHOUGH PART OF THE REQUEST WAS AGAIN BLOCKED BY ANOTHER XMISSION. OUR TCAS SUDDENLY RPTED A CONFLICT AND I DISENGAGED THE AUTOPLT AND DSNDED BELOW OUR ASSIGNED FLT LEVEL UNDER AN ASSUMPTION THAT ANOTHER AIRPLANE WAS DSNDING INTO OUR FLT LEVEL. I CANNOT RECALL HOW FAR BELOW ASSIGNED ALT; BUT I BELIEVE IT WAS LESS THAN 1000 FT. THERE WERE NO ACFT SHOWING ON THE DISPLAY BELOW US. ONCE WE SHOWED THE CONFLICT BEHIND US; I RE-ENGAGED THE AUTOPLT AND CLBED BACK TO THE ASSIGNED ALT. WE ASKED THE CTLR WHAT HAPPENED BUT WAS UNCLR IN THE RESPONSES. I THINK A SUPVR MAY HAVE BEEN ON THE FREQ AS THERE MAY HAVE BEEN 2 DIFFERENT VOICES. I ALSO HEARD THE OTHER ACFT RPT TO THE CTLR THAT THEY HAD US IN SIGHT DURING THE XING OF PATHS (HE DID NOT INDICATE HOW CLOSE WE WERE). I DO NOT KNOW WHAT THE SEPARATION FROM THE OTHER ACFT MAY HAVE BEEN AND WHETHER WE REACHED A CRITICAL CONFLICT SIT.