A FLC FLEW A CLRNC INTENDED FOR ANOTHER COMPANY ACFT NEAR SPS.

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WE WERE LEVEL AT 10000 FT MSL; ZFW CLRED OUR FLT ACFT X TO 15000 FT. WE READ BACK THE CLRNC TO CLB TO 15000 FT. AS WE WERE CLBING THROUGH 11000 FT; ATC ASKED US TO SAY OUR ALT. WE TOLD HIM AND HE SAID 'YOU TOOK FLT ACFT Y'S CLB CLRNC; LEVEL AT 12000 FT.' 5 OR SO MINS LATER; ATC CLRED US TO CLB TO FL210. OUR COMPANY ACFT; FLT ACFT Y QUERIED THE CTLR TO SEE IF THE CLB CLRNC WAS FOR ACFT Y OR ACFT X. THE CTLR; REALIZING HIS MISTAKE; TOLD US ACFT X NOT TO CLB AND THEN CORRECTLY ISSUED ACFT Y'S CLB CLRNC TO FL210. WE WERE ONLY FILED TO 16000 FT FOR CRUISE. TWICE WE RECEIVED INCORRECT CLB CLRNCS FROM THE CTLRS. THE PROB FOR BOTH PLTS AND CTLRS IS THAT AIRLINES HAVE WAY TOO MANY SIMILAR FLT NUMBERS SCHEDULED TO OPERATE IN THE SAME AIRSPACE EACH DAY. WITH A HUB AND SPOKE OP; CLOSELY NUMBERED FLTS TO AND FROM A CITY PASS EACH OTHER IN THE SAME AIRSPACE CAUSING CONFUSION FOR CTLRS AND PLTS. I HAVE ALWAYS BEEN AMAZED AT THE LACK OF THOUGHT PUT INTO DESIGNATING FLT NUMBERS OF ACFT OPERATING AT THE SAME TIME ON THE SAME FREQS.