diff --git a/openbsc/src/libmsc/gsm_04_11.c b/openbsc/src/libmsc/gsm_04_11.c
index bf256c3..20d18a9 100644
--- a/openbsc/src/libmsc/gsm_04_11.c
+++ b/openbsc/src/libmsc/gsm_04_11.c
@@ -780,7 +780,7 @@
 {
 	struct gsm48_hdr *gh = msgb_l3(msg);
 	uint8_t msg_type = gh->msg_type;
-	uint8_t transaction_id = ((gh->proto_discr >> 4) ^ 0x8); /* flip */
+	uint8_t transaction_id = gsm48_hdr_trans_id_flip_ti(gh);
 	struct gsm_trans *trans;
 	int new_trans = 0;
 	int rc = 0;