AN ACR PIC RESPONDS TO A WRONG CALL SIGN AND THE FO; PF; PERFORMS AN ALT EXCURSION.

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WE HAD BEEN CLRED THE CEDAR CREEK 2 ARR. WHILE ON REGIONAL APCH CTL; APPROX 3-4 NM N OF TACKE; WE WERE CLRED TO DSND FROM 11000 FT TO 7000 FT AND SWITCH TO ANOTHER REGIONAL APCH FREQ. I READ BACK THE CLRNC TO 7000 FT; THE NEW APCH FREQ AND OUR CALL SIGN. AFTER CHKING IN WITH THE NEW CTLR; HE ACKNOWLEDGED OUR CHK-IN WITH ACR XA. I RESPONDED WITH ACR AXXA AND OUR ALT CLRNC. HIS RESPONSE WAS ACR XA; WHAT'S YOUR POS. I DIDN'T RESPOND TO THE OTHER CALL SIGN SO AS NOT TO CONFUSE THE SIT AND CLUTTER THE RADIO TFC. I WAS WAITING FOR ACR XA TO RESPOND; AND THEN I WOULD CHK-IN AGAIN. HOWEVER; AFTER A FEW SECONDS; THE CTLR SAID; ACR AXXA; CLB IMMEDIATELY TO 11000 FT; AND SWITCH BACK TO YOUR PREVIOUS FREQ. THE FO (PF) HAD DSNDED TO 10300 FT AND THERE WAS TCASII TFC ON THE NOSE ABOUT 8 NM; APPROX 200 FT BELOW OUR ALT; AND DSNDING. AFTER SWITCHING BACK TO OUR ORIGINAL FREQ; I INQUIRED ABOUT THE CONFUSION. THE CTLR SAID IT WAS NO PROB. APPARENTLY; THE TCASII TFC ON OUR NOSE WAS ACR XA. SUPPLEMENTAL INFO FROM ACN 405580: THE CTLR TOLD NOT TO WORRY AND THAT THIS CONFUSION WAS DUE TO SIMILAR SOUNDING CALL SIGNS. OUR LOWEST ALT WAS 10300 FT AND WE DID OBSERVE TFC ON THE TCASII 8 MI ON OUR NOSE 100 FT BELOW US.