Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 233049)
+++ channels/chan_sip.c	(working copy)
@@ -2160,9 +2160,13 @@
 		if (sscanf(p->lastmsg, "Tx: %30s", method_str) == 1 || sscanf(p->lastmsg, "Rx: %30s", method_str) == 1) {
 			if (method_match(SIP_CANCEL, method_str) || method_match(SIP_BYE, method_str)) {
 				ast_set_flag(&p->flags[0], SIP_NEEDDESTROY);
+				return 10000;
 			}
 		}
-		return 10000;
+
+		/* Give them twice the maximum interval to respond, then really kill the dialog. */
+		snprintf(p->lastmsg, sizeof(p->lastmsg), "Tx: CANCEL (autodestruct)\n");
+		return (p->timer_t1 < 500 ? 500 : p->timer_t1) * 64 * 2;
 	}
 
 	/* If we're destroying a subscription, dereference peer object too */
@@ -3116,6 +3120,8 @@
 			ast_queue_control(p->owner, AST_CONTROL_CONGESTION);
 			ast_channel_unlock(p->owner);
 		}
+		ast_set_flag(&p->flags[0], SIP_NEEDDESTROY);
+		p->invitestate = INV_CANCELLED;
 	}
 	ast_mutex_unlock(&p->lock);
 	return 0;