Date: 201311
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Weather elements / visibility: Turbulence
Light: Daylight
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Integrated Audio System
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Human factors: Troubleshooting; Confusion; Communication Breakdown
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Other / Unknown
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Ambiguous

Narrative: During climbout on the second leg passing FL180; we elected to turn the speakers on and remove our headsets. As the pilot monitoring I checked in with the Center High Controller and was given a climb to our filed altitude of FL390. As we continued climbing to our assigned flight level; I noticed a TCAS target about 5;000 FT above us and about 10 miles away at our two o'clock position. This flight crossed in front of our nose about 5 miles ahead and 1;500 feet above us; but the radio was silent; and I thought that strange; so I did a radio check with Center. Immediately; the Center Controller came back with 'I have been calling you for 2 minutes! Turn 30 degrees right for traffic.' We complied with the vector and the Captain asked me to query him if there was a loss of separation. He curtly replied 'I don't have time right now; I've been trying to reach you for 2 minutes.' Once the radio traffic settled down; I queried him again; and he replied; 'It was close.' On our end; the TCAS target never became solid white and we did not receive a TCAS TA or RA. Prior to removing our headsets; the Captain asked if I minded having the interphone ON to improve inter-cockpit communication. I have never done this before on the B757; but agreed and that is how we operated. However; we did not return the interphone switches to the neutral position when we removed our headsets; and this may have caused our speakers to be inhibited. Once I called Center however; two-way comm. was restored. After the event; the Captain checked in the Aircraft Systems Manual to see if having both Interphone switches ON caused the speakers to be inhibited; but there was no information on this subject. Apparently; however; on this particular ICS panel; if both Interphone switches are ON; the speakers are inhibited; effectively causing a Loss Comm. situation. There are a couple ways: If above FL180; and the crew elects to 'Go speaker' and remove their headsets; they must ensure that both interphone switches are in the neutral position. Secondly; when the crew does 'Go speaker' they should do so during a center transmission to ensure that the speakers are working properly.


Second narrative: We could not understand why we couldn't hear center over the speaker. Finally we deselected interphone and the speaker came alive. I consulted the Aircraft Systems Manual and found that the section describing the interphone switch does NOT say selection of interphone would inhibit the cockpit speaker. I guess inhibiting the speaker with interphone selected makes sense from a feedback standpoint.We didn't realize selection of cockpit speaker with interphone selected would inhibit the speaker. Selecting speaker while Center was transmitting in order to verify speaker operation instead of assuming the speaker is operating normally. In Addendum Systems Manual; a more detailed description of the functioning of interphone switch and a caution that selection of interphone inhibits speaker function.


Synopsis: A B757 crew was unable to hear ATC on the overhead speaker because selecting overhead cockpit speaker and interphone muted radio calls and as a result the flight was NORDO with conflicting traffic.