ARTCC CTLR MISTAKENLY CLRED A B727 CARGO ACFT TO CLB DURING ENRTE DUE TO DISTR OR SEVERAL SAME COMPANY SIMILAR CALL SIGNS.

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FLYING FOR A SCHEDULED CARGO COMPANY; MHR (MATHER; CA) TO PHX (PHOENIX; AZ). CTLR WAS WORKING MULTIPLE SECTORS AND FREQS DURING OUR NIGHTLY PUSH FROM MHR. WE HAVE 9 FLTS DEPARTING WITHIN A SHORT TIME IN VARIOUS DIRECTIONS. OUR FLT (ACR X) WAS ON 127.95 AND TOLD FL290 WOULD BE OUR FINAL ALT. SAME CTLR WAS CONTACTED ON 125.75 (ZOA). AFTER A FEW MINS; WE WERE CLRED TO FL330. AS WE PASSED FL305; CTLR ASKED US FOR OUR ALT. WE RESPONDED 'PASSING FL305; CLBING TO FL330 AS CLRED.' CTLR ADVISED WE WERE SUPPOSED TO MAINTAIN FL290; BUT NO PROB; HE STATED. FO STATED WE HAD BEEN CLRED TO FL330 BY CALL SIGN ACR X; AND HE HAD READ BACK CLRNC 'FROM FL290 TO FL330; ACR X.' CTLR RESPONDED 'YEAH; TOO MANY ACR X CALL SIGNS.' PROB WAS CAUSED BY TOO FEW CTLRS WORKING TOO MANY SECTORS LATE AT NIGHT. COMPLICATING THIS IS OUR NIGHTLY PUSH OF ACFT WITH SIMILAR CALL SIGNS HDG IN MULTIPLE DIRECTIONS. LUCKILY; THERE WAS NO CONFLICT IN THIS INCIDENT. WE WERE OFF OF PUBLISHED AIRWAYS; PROCEEDING DIRECT TO BTY VOR; LATE AT NIGHT. ONE MORE CTLR WORKING AT ZOA DURING THIS TIME PERIOD WOULD BE A TREMENDOUS HELP AS MHR WILL BECOME OUR PERMANENT HUB; DAY AND NIGHT; AND TFC WILL INCREASE. WE COULD/SHOULD HAVE CHALLENGED THE CTLR BECAUSE OF HIS EARLIER STATEMENT; FL290 WILL BE YOUR FINAL.