Date: 201510
Local time of day: 1801-2400
Location: ZHU.Airport, TX
Flight conditions: VMC
Light: Night
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZHU
ATC / advisory: Center ZHU
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Situational Awareness
Second reporter function: First Officer
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Security
Detector: Person Dispatch
When detected: In-flight
Result: Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: While operating Flight to Houston (HOU) I asked ATC to change our call sign from '(call sign XXXX)' to '(call sign XX).'  This was done in an effort to lessen the mental workload of the four digit call sign. ATC obliged and changed it to '(call sign XX).' We were queried by Center as to why we wanted to change our call sign?  I responded to lessen our workload and prevent missed radio calls. Sometime later in the flight; Company Dispatch sent us a message via ACARS asking if we were ok. I replied 'yes; smooth as butter.' Dispatch then sent another message asking where we were going on our next leg. I replied 'ZZZ'.  Dispatch then said to call when we got to HOU because they needed to tell us about a PIREP.  I pulled up the HOU and ZZZ ATIS and NOTAMS via ACARS but didn't find any PIREPS.After safely landing in HOU and when my workload permitted; I called our new Dispatcher. He was unaware of any PIREP message. I then called the desk of our Dispatcher for Flight XXXX to HOU but he had left for the day. The new Dispatcher figured out what was going on and connected me with the Chief Pilot on Duty; who graciously explained the situation and the concerns ATC had with security and possible consequences had we not answered our Dispatcher's query. The Chief Pilot reiterated the importance of our call sign remaining intact for operational necessity as well as the National Security concerns. I was unaware of the severity of the issue and was edified by our Chief Pilot.I apologized for the concerns I had caused to ATC and our Company. I tried to explain that I was only trying to lessen our mental workload because of the basic principles of Psychology regarding short term memory. Phone numbers are 7 digits so we can remember them; Social Security numbers are 9 digits so others can't remember ours. When we have four digit call sign coupled with trying to remember a four digit frequency; our short term memory capable of only 7 numbers on average; becomes taxed. In the future; I will not change our call sign and certainly don't want to raise security concerns for ATC and our Company nor make our Dispatchers duties more difficult. I would only ask for Company Flight Planning to recognize the additional workload caused by four digit call signs and please try to minimize their use.


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


Synopsis: B737 Captain reported difficulties with his attempts to change his call sign to a two-digit number instead of the four-digit number they were dispatched with.