Date: 201512
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: Airbus 318/319/320/321 Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ; Tower ZZZ
Aircraft component: Air/Ground Communication
Component problem: Design
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Distraction; Workload; Communication Breakdown
Second reporter function: First Officer; Pilot Not Flying
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Environment - Non Weather Related; Human Factors; Procedure; Airport
Primary problem: Procedure

Narrative: During our initial climb; I recall another aircraft cleared to line up after us on 18C; and then takeoff; followed by one more aircraft on 18C runway and then also cleared takeoff. Possibly there was additional radio traffic; but I don't recall specifics. There seemed to be some minor scratchy interference; possibly an aircraft attempting a transmission.As we were passing around 2;000 feet; (on the 190 heading; as cleared); we were instructed to contact departure; which the First Officer (FO)/Pilot Monitoring (PM) acknowledged. (It seems to me there may have been a slight pause between our call sign and the contact departure instruction.) The departure frequency had been preloaded onto the radio head; and the FO changed to Departure. On initial contact with Departure; FO checked on with our flight number; heading assignment (don't recall if he said Runway Heading or Heading 190); and 2;500 for 8;000. Departure acknowledged with: Fly heading 280 & Direct. We complied with this clearance. Subsequently; we were assigned a climb to 16;000; and then a handoff to Center. Passing approximately 15;000; Center asked us to take a phone number to call. We accepted the number; and FO/PM inquired what is this regarding? We were told that it was about a possible pilot deviation. We were completely puzzled and began to review our actions on the ground during taxi; taking the runway; initial takeoff roll; initial climb out; and all the configurations and hand-offs during that process preceding the phone number request. We took notes regarding all this; and reviewed our clearance; release and weather ATIS; but neither the FO nor I could recall any issue that would account for a pilot deviation. There had been no concern or inquiry from any controller; no TCAS alert; no solid diamond on our Navigation Display to indicate traffic closure. Visibility was good (greater than 10 miles). We were baffled but recorded our recollections nevertheless.Upon arriving in our destination; the FO and I conferred by phone with Tower Supervisor who was most courteous and direct in explaining why we had been asked to call. The Supervisor said a BRASHER report had been generated because at approximately XA:48; we had ignored a heading assignment of 240 issued by Tower; and that this had resulted in a possible loss of separation with an aircraft on climb from Runway 18L. According to Tower Supervisor; the 240 heading clearance had been issued by Tower controller and had been issued in the same transmission as the assignment to contact departure. Tower Supervisor further stated that the controller tried to get us back; but that were already off the Tower frequency.The First Officer and I were completely surprised and even astounded by this allegation; because neither of us remembered this instruction; although we do remember the 'contact departure'.  I asked the Supervisor if the tapes show that we read back the 240 heading; knowing the read back would NOT be on the tapes. The[n] I attempted to clarify that the Tower Controller could NOT have received our acknowledgement/read back of the 240 heading assignment because we did not ever hear such a 240 heading clearance in the first place. I added that absolutely if we had indeed heard an amended clearance heading 240; a read back and compliance would have been immediate. Supervisor only said that there was a possible loss of separation and explained a few of the details.  Then the First Officer spoke briefly with [the] Supervisor; after that we asked about the next steps in the process. Again; Tower Supervisor was straightforward and professional in communicating the subsequent actions we could expect; and offered to make the tapes available to either of us; directing us to the Operations Manager of the Tower facility.  At the conclusion of the trip when I was back at that airport; Tower Operations Manager and I reviewed the tapes and the video presentation of the 3-5 minutes applicable to this issue. Asa comment here; I have to say I have never before had cause to be in a situation where I reviewed recordings of ATC/Aircraft radio transmissions. I was most surprised at the clarity of the recording because the transmissions WE receive on our radios when we are flying are often accompanied by background noise from the transmitting side; scratchy white sounds; other aircraft blocked transmissions and so forth; not to mention the ambient sounds of ventilating fans and gear snubbers etc. from the cockpit side. In the review in a quiet office; the assignment ' [Aircraft X] fly heading 240 and (then?) Contact departure' was definitely on the tape. I could not have been more surprised. I know that the First Officer nor I heard that heading 240 clearance; not had the First Officer acknowledged/read back the 240 heading assignment. We spoke for awhile about the event; and about local ATC procedure. We discussed the potential follow up actions from the District Office and FSDO. The Manager encouraged us to file reports.  I appreciate that Operations Manager took time to make this meeting possible; and I thank him for being available to answer my questions and clarify my concerns.  I can only say; in conclusion; that I believe that pilots and controllers are partners; if you will; and that we have a serious responsibility and an important common goal of safe flight. I would never ever knowingly ignore a clearance from a traffic controller. Depending on our roles for that flight; either I or my First Officer would always promptly read back any clearance; and comply immediately. I see several contributing factors to this event; all of which are beyond by my personal control.1) The heading clearance was issued concurrent with a frequency change; without clear confirmation of that heading clearance; and no opportunity to resolve it because a frequency change was issued too.2) Considerable radio chatter and possible blocked transmission or white noise possibly interfered with radio transmission. (Unavoidable at a busy airport)3) The RNAV SID does not address traffic divergence; but relies on ATC instructions for separation. This is less of a factor for this specific issue; because we were not following the published instructions of the RNAV SID; but rather; we were following our last assigned clearance (heading 190).A CLEARANCE UNACKNOWLEDGED IS A CLEARANCE NOT HEARD.In an effort to avert future issues such as ours; where an issued clearance is partially 'lost'; I have two recommendations; and one vow.1) Separate any clearances from a frequency change. Allow for accurate clearance read back and any resolution BEFORE assigning the frequency change; and thus minimize any opportunity for frequency blockage; misunderstood clearances; or 'lost 'clearances.2) Revisit the RNAV SID to address and include diverging paths; and not rely only on ATC verbal instruction to ensure critical traffic separation. (Although this doesn't completely apply to us because we were in fact on a heading assignment not depicted on the published RNAV SID; I still think that if subsequent divergent turns were specifically drawn into the RNAV SID then situations such as ours would be less likely ).3) As for me; although I believe suggestion #1 to be a more conservative safe procedure; now knowing that in common daily practice local ATC will issue a two-step clearance to include a frequency change; to the best that the situation allows; (notwithstanding radio interference; transmission blockage and/or emergency etc.); moving from here forward I vow that I myself will be on even more heightened alert so as to expect this dual step hybrid clearance; and additionally; I will pause before leaving the frequency... just in case.


Second narrative: [Report narrative contained no additional information.]


Synopsis: An air carrier departed a Class B airport flying an RNAV SID but because of communication congestion missed a heading clearance issued concurrently with a frequency change. An airborne conflict resulted; but the RNAV SID contained no heading guidance beyond an initial heading.