diff --git a/include/osmocom/msc/call_leg.h b/include/osmocom/msc/call_leg.h
index 61107ee..d8380f5 100644
--- a/include/osmocom/msc/call_leg.h
+++ b/include/osmocom/msc/call_leg.h
@@ -34,6 +34,7 @@
 	/* Array indexed by enum rtp_direction. */
 	enum mgcp_connection_mode crcx_conn_mode[2];
 
+	/* Events dispatched to the parent fi, see call_leg_alloc() doc. */
 	uint32_t parent_event_rtp_addr_available;
 	uint32_t parent_event_rtp_complete;
 
diff --git a/src/libmsc/call_leg.c b/src/libmsc/call_leg.c
index 1b7db2b..46405bc 100644
--- a/src/libmsc/call_leg.c
+++ b/src/libmsc/call_leg.c
@@ -64,6 +64,14 @@
 	OSMO_ASSERT( osmo_fsm_register(&call_leg_fsm) == 0 );
 }
 
+/* Allocate a call leg FSM instance as child of an arbitrary other FSM instance.
+ * The call leg FSM dispatches events to its parent FSM instance on specific events:
+ * - parent_event_term: dispatch this to the parent FI when the call leg terminates (call ended, either planned or by
+ *   failure).
+ * - parent_event_rtp_addr_available: one of the rtp_stream instances managed by the call leg has received an RTP
+ *   address from the MGW. The struct rtp_stream instance is passed as data argument for the event dispatch.
+ * - parent_event_rtp_complete: one of the rtp_stream instances entered the RTP_STREAM_ST_ESTABLISHED state.
+ */
 struct call_leg *call_leg_alloc(struct osmo_fsm_inst *parent_fi,
 				uint32_t parent_event_term,
 				uint32_t parent_event_rtp_addr_available,