diff -Nurp -x .svn a/channels/chan_sip.c b/channels/chan_sip.c
--- a/channels/chan_sip.c	2009-09-15 01:49:22.859014584 -0400
+++ b/channels/chan_sip.c	2009-09-15 01:51:23.515075235 -0400
@@ -18719,16 +18719,18 @@ static void handle_response(struct sip_p
 		pbx_builtin_setvar_helper(owner, causevar, causeval);
 	}
 
-	/* Acknowledge whatever it is destined for */
-	if ((resp >= 100) && (resp <= 199)) {
-		ack_res = __sip_semi_ack(p, seqno, 0, sipmethod);
-	} else {
-		ack_res = __sip_ack(p, seqno, 0, sipmethod);
-	}
+	if (p->socket.type & SIP_TRANSPORT_UDP) {
+		/* Acknowledge whatever it is destined for */
+		if ((resp >= 100) && (resp <= 199)) {
+			ack_res = __sip_semi_ack(p, seqno, 0, sipmethod);
+		} else {
+			ack_res = __sip_ack(p, seqno, 0, sipmethod);
+		}
 
-	if (ack_res == FALSE) {
-		append_history(p, "Ignore", "Ignoring this retransmit\n");
-		return;
+		if (ack_res == FALSE) {
+			append_history(p, "Ignore", "Ignoring this retransmit\n");
+			return;
+		}
 	}
 
 	/* If this is a NOTIFY for a subscription clear the flag that indicates that we have a NOTIFY pending */