Index: kernel/generic/include/ipc/ipc.h
===================================================================
--- kernel/generic/include/ipc/ipc.h	(revision 6df41418d0196f04b16391c56883d36325ab4b0d)
+++ kernel/generic/include/ipc/ipc.h	(revision c377c5e474885d33e24101d261afc20e99e62460)
@@ -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;