diff --git a/src/v5x_data.c b/src/v5x_data.c
index 020f450..ade95b0 100644
--- a/src/v5x_data.c
+++ b/src/v5x_data.c
@@ -350,7 +350,8 @@ void v5x_interface_free(struct v5x_interface *v5if) {
 struct v5x_user_port *v5x_user_port_create(struct v5x_interface *v5if, uint16_t nr, enum v5x_user_type type,
 	uint8_t ts1, uint8_t ts2)
 {
-	struct v5x_user_port *v5up;
+	struct v5x_user_port *v5up, *tmp;
+	struct llist_head *add;
 	int i;
 	int rc;
 
@@ -388,7 +389,16 @@ struct v5x_user_port *v5x_user_port_create(struct v5x_interface *v5if, uint16_t
 	if (!v5up)
 		return NULL;
 
-	llist_add_tail(&v5up->list, &v5if->user_ports);
+	/* Insert in a sorted way: all port numbers in ascending order, but all ISDN ports behind PSTN ports */
+	add = &v5if->user_ports;
+	llist_for_each_entry(tmp, &v5if->user_ports, list)  {
+		if (tmp->type == V5X_USER_TYPE_ISDN && type == V5X_USER_TYPE_PSTN)
+			break;
+		if (tmp->nr > nr && tmp->type == type)
+			break;
+		add = &tmp->list;
+	}
+	llist_add(&v5up->list, add);
 
 	v5up->interface = v5if;
 	v5up->nr = nr;