FLC OF LGT ACCEPTS WRONG DSCNT CLRNC AND RECEIVES TCASII TA THEN RA. ATC ALSO RECOGNIZES THE PROB AND COMMUNICATES WITH FLC TO RETURN TO ASSIGNED ALT. THERE ARE SIMILAR SOUNDING ALPHA NUMERICS ON THE FREQ.

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I WAS FLYING FROM PDX TO SAN. IT WAS THE CAPT'S LEG TO FLY; BUT I WAS ALONE ON THE FLT DECK AS THE CAPT WAS ATTENDING TO PHYSIOLOGICAL NEEDS. ABOUT 30 MI N OF LAX AT FL330; WHILE PROCEEDING DIRECT TO LAX; I RECEIVED A CLRNC FROM LAX TO DSND TO FL290. I READ BACK THE CLRNC; MY CALL SIGN AND WITH NO FURTHER COM FROM CTR; BEGAN A DSCNT. AT THAT POINT; THE CAPT RETURNED TO THE FLT DECK. JUST PRIOR TO; OR AS HE RE-ENTERED THE FLT DECK; I RECEIVED A TCASII TFC ALERT. THE SYMBOL WAS AT THE 12 O'CLOCK POS INDICATING '-10' AND NEITHER THE POS NOR ALT WAS CHANGING EVEN THOUGH WE WERE DSNDING. I INITIALLY CONSIDERED THE POSSIBILITY THAT THIS MIGHT BE A FAULTY WARNING. AS I WAS DECIDING TO STOP THE DSCNT UNTIL THIS COULD BE SORTED OUT; I RECEIVED A TCASII RA AND IMMEDIATELY COMPLIED. THE CTLR THEN INQUIRED IF WE WERE LEVEL AT FL330 AND I REPLIED THAT WE WERE NOT. HE THEN GAVE ME A CLRNC BACK TO FL330. WE VISUALLY ACQUIRED THE OTHER ACFT AT 2-3 MI AS HE PASSED OFF OUR R SIDE. CONTRIBUTING TO THIS PROB WAS THE PRESENCE OF A SIMILAR SOUNDING CALL SIGN ON THE SAME FREQ. I WAS UNAWARE OF THEIR PRESENCE PRIOR TO ACCEPTING THE DSCNT CLRNC. BOTH THE CTLR AND MYSELF WERE USING THE CALL SIGN 'COMPANY EFD' AND WHEN SPEAKING LIKE THIS; 'EED' IS PHONETICALLY VERY CLOSE. EVEN WITH CLOSE ATTN TO DETAIL; GIVEN THE NOISE LEVEL ON THE FLT DECK AND THE LOSS OF SPEECH QUALITY OF RADIO XMITTED VOICES; IT IS POSSIBLE I MAY HAVE ACCEPTED EED'S DSCNT CLRNC. JUST BEFORE RECEIVING THE TCASII TA; I HEARD CTR INSTRUCT EED TO BEGIN THEIR DSCNT. I DID NOT HEAR THE ENSUING DISCUSSION DUE TO THE TCASII ADVISORY. SINCE EED DID NOT RESPOND TO THE INITIAL DSCNT; AND DID NOT QUESTION MY READBACK OF THE CLRNC (NOR DID CTR); I CONCLUDE THAT THEY ALSO THOUGHT THE CLRNC WAS FOR US. CONSIDERATIONS AND RECOMMENDATIONS: 1) HAD THE CTLR AND I BEEN USING PROPER PHRASEOLOGY; THIS PROB WOULD BE MUCH LESS LIKELY TO HAPPEN. 2) THE RAPIDITY OF SPEECH THAT WE ALL USE BY NECESSITY; AS WELL AS THE VOICE QUALITY OF RADIO XMISSIONS CONTRIBUTES TO THE PROB. 3) I SHOULD HAVE IMMEDIATELY STOPPED THE DSCNT AND VERIFIED THE CLRNC WHEN THE TCASII ADVISORY OCCURRED. 4) WE NEED TO BE MADE AWARE OF SIMILAR SOUNDING CALL SIGNS WHEN WE CHK ON A NEW FREQ.