Index: kernel/generic/include/ipc/ipc.h
===================================================================
--- kernel/generic/include/ipc/ipc.h	(revision cdc433468942845e697cf19ce654c95c544c367e)
+++ kernel/generic/include/ipc/ipc.h	(revision e618885aeebd9059e7aa066703b5c59019620a4e)
@@ -82,4 +82,9 @@
 	waitq_t wq;
 
+	/**
+	 * Number of answers the answerbox is expecting to eventually arrive.
+	 */
+	atomic_t active_calls;
+
 	/** Phones connected to this answerbox. */
 	list_t connected_phones;