Date: 201812
Local time of day: 1801-2400
Location: EBBU.ARTCC, FO
Flight conditions: VMC
Aircraft: Widebody Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Descent; Cruise
ATC / advisory: Center EBBU
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Communication Breakdown
Second reporter function: Pilot Flying; First Officer
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: Handed off from Paris Controller (who had a thick accent.) What I understood and read back was '...contact Brussels on 125.6'. The pilot flying thought he heard 125.0. Because the Controller didn't correct my read back I checked in with Brussels on 125.6 The Controller acknowledged immediately (leading us to believe that 125.6 was the correct frequency) and what I heard and thought he said was 'descend to FL180'. The pilot flying heard the same and initiated a descent to FL180. I read back 'leaving FL230 for FL180.' As we were descending the Controller queried us and asked if we had started our descent. I replied 'affirmative; descending through FL220 for 180'. Controller then said to reset transponder as he showed us still at FL250.Realizing something wasn't right I instructed the pilot flying to push altitude hold. The Controller realized something wasn't right at the same time and asked for our position and whether we were on 125.6. Controller told us to maintain FL215 and explained he thought we were [another company flight]; who subsequently checked on to the frequency moments later at FL250. The Controller said there was no conflict and handed us off to 125.0 (Maastricht; I think). It was very hard to discern the difference between [the flight numbers].English not primary language of Controller. Controller not speaking slowly and concisely. My failure to resolve immediately with the Paris Controller the discrepancy between what I heard and what the FO (First Officer) heard (125.6 vs 125.0). Internationally it's sometimes a group effort to discern clearances. In this case it didn't work because the Controller on the wrong frequency was expecting an aircraft with a similar call sign.Reminding Controllers to speak clearer and slower with foreign crews. Any time there is a discrepancy; immediately resolve it before continuing.


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


Synopsis: Air Carrier flight crew reported an altitude deviation in European airspace when they took a clearance to descend intended for another aircraft with a similar call sign. Flight crew also stated language barrier issues contributed to the event.