diff -u -r1.677 chan_sip.c
--- chan_sip.c	17 Mar 2005 23:03:07 -0000	1.677
+++ chan_sip.c	19 Mar 2005 18:10:22 -0000
@@ -3320,7 +3322,7 @@
 		copy_all_header(resp, req, "Record-Route");
 	copy_header(resp, req, "From");
 	ot = get_header(req, "To");
-	if (!strstr(ot, "tag=")) {
+	if (!strcasestr(ot, "tag=") && strncmp(msg, "100", 3)) {
 		/* Add the proper tag if we don't have it already.  If they have specified
 		   their tag, use it.  Otherwise, use our own tag */
 		if (!ast_strlen_zero(p->theirtag) && ast_test_flag(p, SIP_OUTGOING))