B737 Next Generation Undifferentiated
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Total 14000
Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Returned To Departure Airport
Human Factors; Procedure
The Captain and I had just worked a flight and were on a 2:15 hour sit before our next flight. Crew Schedule called my cell phone halfway through our sit time and I let the call go to voicemail. I listened to the message which advised me to check my schedule and confirm that I had a reassignment. I pulled up my [schedule] and saw that the reassignment departed in 35 minutes and consisted of a [Caribbean] turn. My current bid status is 737 Domestic; so the international turn raised a red flag for me. My previous bid status from 5 years earlier had been 737 International. My first call was to [Union] Contract Administration to have them look at the assignment and check for my legality and the legality of the trip assignment. After a few minutes of review; Contract Administration advised that the assignment was legal; and that Crew Schedule could; in fact; reassign me. Since I had been removed from my original sequence; I decided I needed to check in with Crew Schedule. On initial check in; Crew Schedule advised that they had reassigned me the [Caribbean] turn. I; once again; questioned the legality of the assignment and advised them I was a Domestic pilot. Crew Schedule assured me that I was searching legal for the trip; and that they 'wouldn't have been able to give you the assignment if you weren't legal'. At that point; I wondered if my International Qualification distance learning modules that I had completed during the past year had; in fact; made me legal to take the flight. Since I had received confirmation from two sources that I was legal; I decided not to turn down the trip and we departed. Approximately one hour into the flight; we received a message from Dispatch notifying us that they were receiving calls that the First Officer was not qualified for the trip and that we needed to return to ZZZ. We advised ATC that we needed to return to ZZZ due to an international crew legality issue. We then returned uneventfully back to ZZZ.Cause: The scheduling system gave the impression to multiple people that I was qualified; legal; and available for the flight. The time crunch to get the flight out didn't leave enough time to catch the error prior to departure.
B737 Domestic First Officer reported an improper reassignment to an international flight between previously assigned domestic trips.