B737 Next Generation Undifferentiated
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 384; Flight Crew Type 7500
Communication Breakdown
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Clearance
Human Factors; Procedure
We were at FL260 for cruise as it was advertised by a Center as a smoother ride. A subsequent ZZZ Center frequency cleared us to cross the ZZZZZ fix at FL180. ZZZZZ is not part of the RNAV Arrival. It was part of our cruise route of flight. Once sent over to ZZZ Center frequency; I reported 'Flight XXXX FL260; and cleared to cross ZZZZZ at FL 180.' ZZZ Center acknowledged checking on. A few moments later; the same ATC Controller said 'Flight XXXX; descend NOW to FL210.' The Pilot Flying and I both agreed this was a new cruise at FL210 and agreed we still needed to cross ZZZZZ at FL180. Due to frequency congestion; I could not get in a confirmation call that we were to continue to still cross ZZZZZ at FL180 until we were starting the descent out of FL210. At FL205; I did get a call in confirming our clearance to cross ZZZZZ at FL180. ZZZ Center said it was ok now for our descent; but a few seconds before that was not what he understood we were cleared. He later asked why we thought the ZZZZZ crossing was still in effect. I stated it was the 'descend NOW to FL210' (from FL260) that made it seem like a new cruise altitude was being assigned to the original ZZZZZ crossing.
B737 Captain reported an altitude deviation due to a communication breakdown with ATC.