B737 captain reported an altitude deviation due to a communication breakdown with ATC.

Attributes
ACN	1578410
Time
Date	201809
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Make Model Name	B737 Next Generation Undifferentiated
Operating Under FAR Part	Part 121
Flight Phase	Cruise Descent
Flight Plan	IFR
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 384 Flight Crew Type 7500
Events
Anomaly	ATC Issue All Types Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Published Material / Policy Deviation - Procedural Clearance

Narrative:

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.

Original NASA ASRS Text

Title: B737 Captain reported an altitude deviation due to a communication breakdown with ATC.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.