CONFUSION WITH SIMILAR SOUNDING CALL SIGNS. A SEEMINGLY APPROVED DSCNT TURNED OUT TO BE AN ALT EXCURSION WHEN THE DESIGNATED RECIPIENT FOR A DSCNT CLRNC WAS NOT DEFINED BY THE CTLR AT ZAB; NM.

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DURING FLT TO SDL; 2 OF OUR COMPANY ACFT WERE OFTEN ON SAME CTR FREQ. SINCE WE HAVE SIMILAR SOUNDING CALL SIGNS; IT IS OUR COMPANY POLICY TO READ BACK FULL CALL SIGNS. AS WE APCHED 1 HR OR SO FROM DEST; A ZAB CTLR ISSUED A CLRNC FOR ABZXY TO DSND. SINCE BOTH OF OUR ACFT HAVE NUMBERS THAT END IN ZXY; I ASKED; USING MY FULL CALL SIGN; IF THAT WAS FOR US. I WAS TOLD IT WAS AND ACCEPTED THE CLRNC USING OUR FULL CALL SIGN ONCE AGAIN. ABOUT 500 FT INTO THE DSCNT; OUR SISTER ACFT ASKED AGAIN FOR CLARIFICATION AND WAS TOLD THE CLRNC WAS FOR THEM. WE IMMEDIATELY ANNOUNCED THE PROB TO ATC AND CLBED BACK TO ALT. UPON REACHING; I CALLED CTR AND REMINDED HIM THAT I HAD ASKED FOR CLARIFICATION AND HAD BEEN TOLD THE CLRNC WAS FOR US. A DISCUSSION ABOUT SIMILAR CALL SIGNS ENSUED. IN OUR PRESENT ATC SYS; SIMILAR CALL SIGNS CONTINUE TO BE AN ISSUE WHETHER COMMERCIAL OR PVT; BUT I FEEL IN THIS INSTANCE THAT WE DID WHAT WE COULD TO CONFIRM WITH CTR THAT THE CLRNC WAS OURS. HE DID NOT DIFFERENTIATE BTWN THE 2 ACFT AND THUS THE PROB OCCURRED. TO MY KNOWLEDGE THERE WAS NO ALT OR SPACING CONFLICTS THAT RESULTED FROM THE MISCOM.