Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 342984)
+++ channels/chan_sip.c	(working copy)
@@ -18391,6 +18391,13 @@
 			per device. I don't want incoming callers to record calls in my
 			pbx.
 		*/
+		
+		if (!p->owner) {        /* not a PBX call */
+                        transmit_response(p, "481 Call leg/transaction does not exist", req);
+                        sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
+                        return;
+                }
+
 		/* first, get the feature string, if it exists */
 		struct ast_call_feature *feat;
 		int j;