Regional Jet 700 ER/LR (CRJ700)
Flight Crew Air Transport Pilot (ATP)
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance
Human Factors
We were in cruise flight at FL340. I was the pilot flying. Both pilots 'heard' clearance for [Air Carrier] XXXX to descend to FL330. The First Officer read back the clearance and a descent was initiated for 330. Almost immediately after starting the descent; I heard ATC call an [Air Carrier] XXYX with a descent to 330. I immediately stopped our descent and started back up to 340. We had descended about 400 FT below 340. We then verified our cleared Flight Level as 340. The flight continued without further incident.1. Primary threat in this case seems to be two extremely similar call signs on the same frequency. As of this writing neither pilot not flying nor pilot flying is sure if we misheard the call sign or not. Also; when we read back the clearance there was no correction from Center; the Controller may not have heard the call sign correctly either. Clearly there was a comm issue between the flights and ATC; both of us believed we heard the appropriate callsign for the clearance.  This is a reoccurring issue with multiple callsigns all starting with '[Air Carrier] 4XXX or 5XXX. It seems absurd that we continue to create this call sign confusion simply because of the way a reservations computer categorizes flights.I am already very vigilant with ATC comm and altitude verification; however; it would appear that even more scrutiny is required; especially near hub airports. 2. We have a four digit call sign. Why we have to have '[Air Carrier] XXXX' and '[Air Carrier] XXYX' in the same airspace at the same time is beyond me. This threat is created and perpetuated by the current system of numbering flights.
We are not sure if we read back another flight's clearance or if the Controller had us switched. We read the clearance back as our own and ATC did not correct or change what we read back.
A CRJ-700 accepted a descent clearance intended for another company aircraft with a similar call sign. The error was detected and corrected after less than a 500 foot excursion. ATC did not recognize the readback error.