B757-200
Flight Crew Air Transport Pilot (ATP)
Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Took Evasive Action; Flight Crew FLC complied w / Automation / Advisory
Human Factors
While at cruise at FL380; ATC informed us to 'turn right 30 degrees; immediately.'  As I confirmed the clearance; the First Officer (Pilot Flying) started the right turn.  While commencing the turn; I saw a co-altitude TCAS target (opposite direction traffic) at approximately the 10-11 o'clock position about 7-8 miles away and closing.  This was followed by a Traffic Alert (traffic; traffic); which was almost immediately followed by a Resolution Advisory (Climb; Climb). The autopilot was immediately disconnected and a TCAS climb initiated.  We climbed approximately 300-400 feet before we were advised 'Clear of Conflict;' and descended back to FL380.After the event; Maastricht Control offered an explanation.  He stated that the conflicting traffic had been on a different frequency and answered someone else's radio call to begin a descent. The conflicting aircraft erroneously started a descent to/through our altitude.   Listening more attentively to ATC when given new altitude clearance. (conflicting A/C)  Had ATC recognized the descending Aircraft earlier perhaps an earlier heading correction for us could have prevented the RA.  Everything happened in an extremely compressed period of time (approximately 30 seconds).  Had we not followed ATC and TCAS instructions immediately the outcome could have been much different
A B757-200 Captain reported receiving an RA from opposite direction traffic at FL380 in oceanic airspace from an aircraft that descended without clearance.