Date: 201306
Local time of day: 1201-1800
Location: ZHU.ARTCC, TX
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Cruise
Airspace: Class A ZHU
ATC / advisory: Center ZHU
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Confusion; Communication Breakdown
Second reporter function: Captain
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Air Traffic Control; Person Air Traffic Control
Result: Air Traffic Control Issued New Clearance
Contributing factors: Procedure; Human Factors; Company Policy
Primary problem: Human Factors

Narrative: Air Carrier X ABCD; a departure from HOU; was cleared to FL390 by me. During the pilot's readback; there was another transmission from another aircraft which blocked Air Carrier's readback. What I heard was another aircraft departing the Houston terminal area checking in climbing to FL230. What I did not hear was Air Carrier X ABED; who was also on frequency; taking Air Carrier X ABCD's clearance. I had not advised these two flights that similar sounding call signs were on the same frequency. Satisfied that the blocking transmission was the other aircraft coming from low altitude I went about my scan. I saw that Air Carrier X ABED was 500 FT high about the same time that the conflict alert fired. I did not question if the aircraft was actually climbing or if I had simply gotten a bad Mode C report for a hit or two and opted to wait another hit or two to see which it was. Once I realized that the aircraft was indeed climbing; I issued a descent due to the proximity of Air Carrier X ABED and another aircraft already established at FL390. In my estimation; lateral separation was not lost between these two aircraft as the Air Carrier appeared to have sufficient time to establish vertical separation. The pilot apologized for taking company's climb clearance and did say that they too wanted FL390. Once Air Carrier X ABED was safely clear of his traffic at FL390 he was cleared to FL390. Had I had the presence of mind to issue a similar call sign warning this would undoubtedly have not occurred. Had I stated after Air Carrier X ABCD's readback was stepped on that the climb clearance was for that aircraft only then this would not have occurred. The pilot might have questioned an unsolicited climb clearance which was issued without explanation from ATC.


Second narrative: Cruising at FL370; we asked for 390 when we were light enough. Controller said next frequency in about two minutes will be the guy to ask. Switched over and asked for 390 and was given 390. Climbing through 38;200 FT; ATC said the clearance was for Company X ABCD not ABED. At all times we used the proper phraseology and distinctly read back ABED. Controller called us ABED; which is correct. ABCD was blocked earlier in the transmission so we did not even know he was there and were never advised of ABCD being on the same frequency.


Synopsis: ZHU described a conflict event when an Air Carrier accepted a climb clearance intended for another aircraft; the Controller acknowledging his/her failure to issue a similar call sign warning.