FLC OF AN ACR MLG ACFT TOOK EVASIVE ACTION IN RESPONSE TO A TCASII RA RESULTING IN EXCURSION FROM ASSIGNED ALT.

Narrative

ZAU HAD CLRED FLT TO 12000 FT DIRECT TO ORD VOR; ADVISED TO CONTACT CHICAGO APCH ON APPROPRIATE FREQ. I ATTEMPTED SEVERAL TIMES TO CONTACT APCH BUT THE CTLR WAS TALKING CONTINUOUSLY; AND I WAS APPARENTLY BLOCKED ON EACH XMISSION AS THE CTLR NEVER ACKNOWLEDGED MY XMISSION. A TCASII TA APPEARED SUDDENLY AT 3 O'CLOCK SHOWING APPROX 12500 FT; FOLLOWED SHORTLY BY AN RA WHICH WAS FOLLOWED; RESULTING IN A CLB OF APPROX 300 FT BEFORE THE CLR OF TFC ADVISORY WAS GIVEN. THE TFC WAS NOT SIGHTED VISUALLY. I CONTACTED APCH IMMEDIATELY TO ADVISE OF THE RA. THE CTLR HAD A DIFFERENT VOICE; INDICATING A CHANGE OF CTLRS AT THE APCH POS. HE ACKNOWLEDGED MY CALL AND REQUESTED OUR LOCATION; SUGGESTING THAT THE PREVIOUS CTLR HAD EITHER FAILED TO ADVISE HIM OF OUR LOCATION; OR HAD NEVER POSITIVELY IDENTED OUR FLT. THE RA WAS PROBABLY DUE TO OUR REMAINING AT THE CTR ASSIGNED ALT OF 12000 FT; WHILE WE WERE UNABLE TO CONTACT APCH CTL DUE TO THE CONGESTION ON THE FREQ. PLTS FLYING INTO ORD ON A REGULAR BASIS ARE AWARE OF THE CONGESTION ON THE FREQS AND TEND TO WAIT FOR THE CTLR TO CALL THEM IF PREVIOUS XMISSIONS HAVE BEEN UNANSWERED. IN THIS CASE; MY FLT SEEMS TO HAVE SLIPPED THROUGH THE CRACKS; AND TCASII AVOIDED A POTENTIAL NEAR MISS.