Fixed handling of call collision

An establishment is received from AN (access network), while
establishment request was sent to AN. The call is answered.

src/pstn/pstn.c

@@ -1542,8 +1542,15 @@ static void v5_receive(pstn_t *pstn, uint8_t *data, int len)
 		break;
 	case PSTN_EVENT_EST_IND:
 		/* in case of collision, just treat as ACK and proceed outgoing call */
-		if (pstn->state != PSTN_STATE_EST_LE
-		 && pstn->state != PSTN_STATE_NULL) {
+		if (pstn->state == PSTN_STATE_EST_LE) {
+			PDEBUG(DTEL, DEBUG_INFO, " -> Handle establish collision, send ACK.\n");
+			/* send ack message to V5 */
+			v5_est_ack_req(pstn);
+			/* treat as establish acknowledgement from PSTN interface */
+			v5_est_ack_ind(pstn, data, len);
+			break;
+		}
+		if (pstn->state != PSTN_STATE_NULL) {
 			PDEBUG(DTEL, DEBUG_NOTICE, "Received %s message in state %s, ignoring!\n", pstn_event_name(event), pstn_state_name(pstn->state));
 			break;
 		}