ACR MLG OVERSHOT ALT ON CLIMB.

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FLT WAS ACR A XYZ; F/O FLYING ACFT AND LEVEL AT FL280 AFTER STAIRSTEP CLB FROM 6000' OUT OF MCI. BOTH HEARD CLRNC TO FL310. FILED FOR FL350 SO ANOTHER STAIRSTEP SEEMED NORMAL. COMMENCED CLB. AT FL299 CENTER ASKED WHAT OUR ALT WAS AND I RESPONDED. CTLR REPLIED THAT WE HAD TAKEN ANOTHER ACFT'S CLB CLRNC. HE THEN CLRED US ON TO FL310. WE HAD READ BACK OUR CLRNC TO THE CTLR AND HE DID NOT OBJECT. WE HEARD NO OTHER TFC ON THE FREQ AND NO OBJECTION FROM ANY OTHER ACFT. AT THE TIME WE RECEIVED THE CLRNC; I WAS MAKING THE REQUIRED PAX ANNOUNCEMENT FOR TURNING OFF SEATBELT SIGN; BUT WAS STILL MONITORING RADIO. THIS APPEARS TO BE ANOTHER TYPICAL HEARBACK/READBACK INCIDENT. IT WOULD BE INTERESTING TO HEAR THE TAPE TO SEE IF THE CTLR CLIPPED OFF THE COMPANY ID AS SO FREQUENTLY HAPPENS. STAIRSTEP CLB CLRNCS; AS I RECALL; WERE 6000'; 15000'; 18000'; 23000'; 26000'; 28000'. WE LEVELED AT 15000'; 23000' AND 28000'. I SPOKE BY TELEPHONE WITH KC CENTER WATCH SUPVRS WHO TOLD ME THAT THERE WAS NO CONFLICT; 'OR I WOULD HAVE HEARD ABOUT IT BEFORE NOW.' MY QUESTIONS: WHY DIDN'T THE CTLR RECOGNIZE THE ERROR IN OUR READBACK? WHY; IF THERE WAS NO CONFLICTING TFC; WERE WE BEING HELD AT FL280 IN THE FIRST PLACE? THERE WAS AN ACR B XYZ THAT DEPARTED MCI A SHORT TIME AHEAD OF US. WAS IT THAT FLT THE CTLR WAS TALKING TO? WE DID NOT HEAR HIM ON OUR FREQ (133.92). WAS THE CTLR TALKING ON MORE THAN 1 FREQ? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR IS CONCERNED BECAUSE SO MANY CTLRS ARE CLIPPING THEIR XMISSIONS AND MANY TIMES COMPANY NAMES ARE NOT HEARD; JUST FLT NUMBER. AND; UNLESS ONE KNOWS ANOTHER FLT WITH THE SAME NUMBER IS ON FREQ; WITH THE AMOUNT OF TFC THEY ARE HANDLING; THE CTLRS ARE NOT CATCHING THE ERRORS. ANALYST EXPLAINED TO RPTR THAT MANY TIMES BOTH ACFT XMIT AT SAME TIME AND CTLR ONLY HEARS ONE. RPTR FEELS THAT BECAUSE OF CTLR SHORTAGE; CTLRS ARE WORKING TOO MANY FREQS. SUPPLEMENTAL INFO FROM ACN 82366: MCI CENTER SAID; ' XYZ; CLRED TO FL 310.' BOTH CAPT (WHO WAS MAKING A PAX ANNOUNCEMENT) AND I REACHED TO RESPOND--I WON. RESPONDED; 'FL310; ACR B XYZ.'