diff --git a/openbsc/src/libmsc/gsm_04_08.c b/openbsc/src/libmsc/gsm_04_08.c
index 9e70ba9..1a1d6f2 100644
--- a/openbsc/src/libmsc/gsm_04_08.c
+++ b/openbsc/src/libmsc/gsm_04_08.c
@@ -3487,7 +3487,7 @@
 {
 	struct gsm48_hdr *gh = msgb_l3(msg);
 	uint8_t msg_type = gsm48_hdr_msg_type(gh);
-	uint8_t transaction_id = ((gh->proto_discr & 0xf0) ^ 0x80) >> 4; /* flip */
+	uint8_t transaction_id = gsm48_hdr_trans_id_flip_ti(gh);
 	struct gsm_trans *trans = NULL;
 	int i, rc = 0;