diff --git a/openbsc/src/osmo-bsc_nat/bsc_ussd.c b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
index 2905c85..e080905 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_ussd.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
@@ -409,7 +409,7 @@
 
 	proto = gsm48_hdr_pdisc(hdr48);
 	msg_type = gsm48_hdr_msg_type(hdr48);
-	ti = (hdr48->proto_discr & 0x70) >> 4;
+	ti = gsm48_hdr_trans_id_no_ti(hdr48);
 	if (proto != GSM48_PDISC_NC_SS)
 		return 0;