B757 FLC HAS CLRNC TO CLB TO 17000 FT AND FREQ CHANGE TO NEW CTLR WHO COMMANDS THEM TO LEVEL IMMEDIATELY AFTER CHKING THEIR ALT. THEY LEVELED AT 16600 FT. NO APPARENT TFC CONFLICT OR LOSS OF SEPARATION INDICATED.

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ON CLBOUT WHILE LEVEL AT ABOUT 13000 FT; THE DEP CTLR BECAME SATURATED WITH RADIO CALLS AND DIRECTED EVERYONE TO 'SHUT-UP;' AND I BELIEVE HE SAID; 'DO NOT ACKNOWLEDGE FURTHER XMISSIONS.' HE THEN CLRED US TO CLB TO 17000 FT. WE ACKNOWLEDGED ACROSS COCKPIT AND SET 17000 FT IN THE ALT ALERT WINDOW; BEGAN THE CLB; BUT DID NOT READ BACK THE CLRNC TO THE CTLR. HE THEN SENT US TO A NEW FREQ. OUR FIRST CALL WAS '...PASSING 16000 FT FOR 17000 FT.' THE NEW CTLR ASKED OUR ALT. WE ADVISED 16600 FT. HE SAID 'LEVEL IMMEDIATELY.' -- ON THE NEW FREQ. THIS WAS NEVER EXPLAINED TO US. WE WERE LEVEL AT 16600 FT FOR ABOUT 2-3 MINS; BEFORE HE CAME BACK AND CLRED US TO FL200; I BELIEVE. WE WERE LEFT WITH THE DISTINCT IMPRESSION THAT WE HAD 'BUSTED AN ALT' BY THE WAY WE WERE DIRECTED TO 'LEVEL IMMEDIATELY' UPON ARRIVING ON THE NEW FREQ. WE WERE DISTRACTED FOR 2-3 HRS AFTER THIS INCIDENT BY DETERMINING HOW TO HANDLE THIS MATTER. THIS CTLR WAS TOO BUSY. HE SHOULD HAVE BEEN RELIEVED SOONER. THE SECTOR IS TOO EASY TO SATURATE WHEN A 'BANK' OF FLTS LEAVE IAD; ESPECIALLY WHEN OUR COMPANY IDENTS FLTS AS XAB; XBB; XCB; ETC. I WILL NEVER AGAIN NOT ACKNOWLEDGE AN ALT CLRNC; REGARDLESS OF THE CTLR'S DIRECTIONS.