Date: 201306
Local time of day: 0601-1200
Location: NCT.TRACON, CA
Flight conditions: VMC
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight phase: Descent
ATC / advisory: TRACON NCT
Reporter organization: Air Carrier
Reporter function: First Officer
Human factors: Communication Breakdown
Anomaly: ATC Issue All Types
Detector: Person Flight Crew
Result: Air Traffic Control Issued New Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: During descent into OAK on the RAIDR 2 RNAV Arrival; we were proceeding directly to the STIKM fix to cross RAIDR at 7;000 FT when we were transferred to another Controller on frequency 124.8. Upon check-in with this new Controller; we were instructed to proceed directly to the OAK VOR and descend to 6;000 FT. I repeated the instructions clearly with our call sign of Company XXX[1]. The Captain; who was the pilot flying; proceeded to program the FMC as I got my approach chart ready. I then noticed the Captain had not set 6;000 FT into the MCP and I brought this to his attention. He said he did not hear that part of the clearance so rather than have him take my word for it I queried the Controller to verify our clearance altitude. The Controller became very puzzled and asked again who we were and where was our present position. I repeated Company XXX[1] and told him approximately 53 miles from the OAK VOR. He immediately told us to level off and we did just that at about 16;800 FT. He then instructed us to return to the previous Controller; but in doing so called us Company XXX[2]. I again queried him to see if the current instructions to level off and return were for Company XXX[1] and he said yes but did not confirm our true call sign. Then another pilot chimed in and said that the Controller was 'calling them with an X' as well. Apparently; there was a Company XXX[2] on the current frequency. I switched back to the previous Controller; who apologized for the miscommunication and cleared us back direct to STIKM and to descend to 7;000 FT. We did just that and I asked her what the other similar sounding call sign was so that we could be on alert for it as we continued to OAK. She said the other call sign was Company XXX[3]. From what I can surmise from the situation; the Controller in question was mistakenly calling the other Company aircraft Company XXX[2] (as opposed to XXX[3]) and when I checked in as directed (as Company XXX[1]) he gave us the clearance meant for the true Company XXX[3]; which was being misidentified as Company XXX[2] and did not exist. There was never any separation issues with any aircraft and no crossing restrictions exist on the arrival other than the expectation for RAIDR at 7;000 FT. Since we were already proceeding direct to STIKM at the time of the mistaken clearance; there was no more than a 10-degree shift in course as we proceeded direct to the OAK VOR as mistakenly cleared; and then cleared back direct STIKM; after the confusion on the Controller's part was resolved. I believe this incident reinforces the importance of verifying (and re-verifying if necessary) clearances when similar sounding call signs are on the same and adjacent frequencies.




Synopsis: Inbound OAK air carrier described an event where similar call signs were in use on the same frequency; clearances were not verified and general confusion resulted.