ACR MLG FAILS TO INITIATE DSCNT WHEN COMMANDED BY IAH APCH.

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FO WAS THE PF. I WAS ASSISTING AS CAPT; APCH CALLED TFC AT 1-2 O'CLOCK AT 5000 MSL AND 10 MI. WE WERE LEVEL AT 6000 MSL ASSIGNED. WE COULD NOT VISUALLY SEE THE TFC AS WE WERE IN AND OUT OF CLOUDS ABOVE AN OVCST. WE DID SEE THE TFC DEPICTED ON OUR TCASII IN THE CORRECT AREA. WE SO ADVISED APCH WHO; THEN CLRED US TO DSND TO 5000 MSL AND ALSO CLRED OUR TFC TO CLB FROM 5000 TO 7000 MSL. WE WERE CONCERNED ABOUT DSNDING INTO THIS CLBING TFC AND SO WE DELAYED OUR DSCNT. DUE TO RADIO TFC WE COULD NOT ADVISE APCH OF THIS. AFTER ABOUT 15 SECONDS THE CTLR ASKED US IF WE HAD STARTED OUR DSCNT. WE ANSWERED BACK THAT WE WERE NOW STARTING THE DSCNT. CTLR QUICKLY GAVE US A VECTOR 90 DEGS TO THE L TO AVOID THE TFC. SEVERAL OTHER VECTORS WERE GIVEN TO INCREASE SEPARATION. DURING THIS TIME FO ALLOWED OUR SPD TO SLOW FROM 250 KTS TO 210 KTS ANTICIPATING A TURN TOWARD FINAL APCH. APCH THEN ASKED OUR SPD AND NOT TO SLOW BEFORE HE REQUESTED IT. WE DID NOT RECEIVE A RA FROM OUR TCASII -- TO THIS TFC. SUBSEQUENT CONVERSATION WITH THE CTLR AFTER LNDG INDICATED WE CLRED THE TFC LATERALLY BY 3 1/2 NM AND NO CONFLICT OCCURRED. HOWEVER; WE THINK THE CTLR ESTIMATED THERE WAS ADEQUATE SEPARATION TO DSND US TO 5000 WHILE CLBING OUR HEAD-ON TFC TO 7000 MSL. WE THINK THE CTLR SHOULD HAVE ALERTED US TO WHAT PLAN HE HAD IN MIND. HIS VECTORS AROUND THIS TFC DID SOLVE THE SEPARATION PROBLEM; BUT LEFT US IN THE DARK AS TO WHAT HIS PLAN WAS. IT IS POSSIBLE THAT WE COULD HAVE RECEIVED A TA IF WE HAD STARTED OUR DSCNT PROMPTLY TO 5000. WITH THE HEAD-ON TFC. THE CTLR LATER SAID HE WAS CONCERNED THAT WE HAD RECEIVED A TCASII ALERT AND WERE STAYING LEVEL TO SATISFY THE TCASII COMMAND. I ADVISED HIM WE DID NOT RECEIVE AN ALERT. THE SUPVR CONTACTED ME AND THE MATTER WAS RESOLVED WITH NO ACTION PLANNED BY ATC. SUPPLEMENTAL INFO FROM ACN 202304. I DID NOT HEAR ATC CLR US TO 5000 FT NOR DID I HEAR CAPT READ BACK 5000 FT -- BUT IT WAS SET IN ALT ALERT. OUR SOP REQUIRES PF TO STATE ALL CLRED ALTS.