Cessna 402/402C/B379 Businessliner/Utiliner
Situational Awareness; Communication Breakdown
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action
Procedure
On Salt Lake Center and level at 11;000. Thought I was told to contact Salt Lake Center and descend 10;000. I had previously requested 10;000 because I had reached more than 30 min above 10;000. So I swapped frequencies and began descending. At 10;500 Center advised of a low altitude warning; and I climbed back up to 11;000.
C402 pilot reported misunderstanding an altitude clearance; resulting in a descent toward terrain.