NMAC BTWN A B727-200 IN LEVEL FLT AND AN UNKNOWN ACFT AT FL290. THE FLC OF THE B727 CLBED IN RESPONSE TO TCASII RA JUST IN TIME TO AVOID A POSSIBLE COLLISION.

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B727 FLT RSW-CVG ON SCHEDULE; APPROX MACH .80 (NORMAL SPD). SWITCHED UP ZTL AROUND ATL VORTAC; WHILE ON COURSE DIRECT TO VXV AND REST OF FLT PLAN RTE. CHKED IN AT FL290 AND WAS TOLD TO EXPECT FL280 SHORTLY NOW TURN R FURTHER 20 DEG VECTOR. AFTER COMPLETING THAT TURN; APPROX 350 DEG HDG; OUR TCASII VERBALLY ALERTED 'TFC.' IT WAS SET AT 50 NM RANGE AND DISPLAYED '00' IN YELLOW AT 11:30 O'CLOCK POS ABOUT 40 MI OFF OUR NOSE. I FLIPPED THE RANGE TO 5 NM AND ALMOST IMMEDIATELY; WITH THE TCASII SAYING 'TFC;' A YELLOW '00' APPEARED AT ABOUT 11:30 O'CLOCK POS. ALL EYES SCANNED FORWARD. CAPT RADIOED CTR ABOUT TFC; NO RESPONSE. TCASII THEN ALERTED 'CLB.' (I WAS PREDISPOSED FOR THIS AS I HAD MY HANDS ON THE YOKE AND THROTTLES DUE TO CONCERN OVER A '00' TARGET AT 12 O'CLOCK POS ON 5 MI RANGE.) I WOULD GUESS 10 SECONDS FOR THE 'CLB' TO ANNUNCIATE; CLICKED OFF THE AUTOPLT; PULLED BACK ON THE YOKE AND POS THE THROTTLES UP 1; VSI NOW READ 1800 FPM UP AND ABOUT FL293 I SAW A SHADOW FLASH OUT MY SIDE (R) WINDOW. WE TOPPED AT FL303. CONTACTED CTR (3 TRIES) TO RPT OUR MANEUVER AND ASK WAS THAT A 'REAL' TARGET. HE RELUCTANTLY SAID IT WAS AND WE WERE NOW CLRED BACK TO FL290. (THE SEAT BELT SIGN WAS ON; AND FLT ATTENDANTS RPTED NO PAX SEEMED AWARE OF THE MANEUVER.) CTR NEVER SEEMED OVERLOADED -- BUT I THINK HE THOUGHT HE HAD GIVEN US FL280 INSTEAD OF 'EXPECT.' I'M NOT SO SURE 'EXPECT' PHRASING HELPS ANYONE -- EXCEPT FOR LATE XING RESTRS. GLAD THAT TCASII DID IT'S JOB.