diff --git a/src/gsm/gsm0808_utils.c b/src/gsm/gsm0808_utils.c
index d8c275a..efa9305 100644
--- a/src/gsm/gsm0808_utils.c
+++ b/src/gsm/gsm0808_utils.c
@@ -510,7 +510,7 @@
 	case GSM0808_CHAN_DATA:
 		byte = ct->data_rate;
 
-		if (ct->data_transparent)
+		if (!ct->data_transparent)
 			byte |= 0x40; /* Set T/NT */
 
 		if (ct->data_rate_allowed_is_set) {
@@ -580,7 +580,7 @@
 	case GSM0808_CHAN_DATA:
 		byte = *elem;
 		elem++;
-		ct->data_transparent = byte & 0x40; /* T/NT */
+		ct->data_transparent = !(byte & 0x40); /* T/NT */
 		ct->data_rate = byte & 0x3f;
 
 		/* Optional extension for non-transparent service */
diff --git a/tests/gsm0808/gsm0808_test.c b/tests/gsm0808/gsm0808_test.c
index 9a6de1e..d7c278f 100644
--- a/tests/gsm0808/gsm0808_test.c
+++ b/tests/gsm0808/gsm0808_test.c
@@ -1098,7 +1098,7 @@
 	struct gsm0808_channel_type dec_ct = {};
 	struct msgb *msg;
 	uint8_t ct_enc_expected[] = { GSM0808_IE_CHANNEL_TYPE,
-		0x03, 0x02, 0x0b, 0x51,
+		0x03, 0x02, 0x0b, 0x11,
 	};
 	uint8_t rc_enc;
 	int rc_dec;
@@ -1142,7 +1142,7 @@
 	struct gsm0808_channel_type dec_ct = {};
 	struct msgb *msg;
 	uint8_t ct_enc_expected[] = { GSM0808_IE_CHANNEL_TYPE,
-		0x05, 0x02, 0x0b, 0x91, 0x8b, 0x20,
+		0x05, 0x02, 0x0b, 0xd1, 0x8b, 0x20,
 	};
 	uint8_t rc_enc;
 	int rc_dec;
@@ -1237,17 +1237,17 @@
 	OSMO_ASSERT(rc == -ENOTSUP);
 
 	/* Data: ext in Octet 5 with transparent service */
-	const uint8_t hex2[] = { 0x02, 0x0b, 0xc0, 0x00 };
+	const uint8_t hex2[] = { 0x02, 0x0b, 0x80, 0x00 };
 	rc = gsm0808_dec_channel_type(&ct, hex2, sizeof(hex2));
 	OSMO_ASSERT(rc == -EINVAL);
 
 	/* Data: ext in Octet 5, but too short */
-	const uint8_t hex3[] = { 0x02, 0x0b, 0x80 };
+	const uint8_t hex3[] = { 0x02, 0x0b, 0xc0 };
 	rc = gsm0808_dec_channel_type(&ct, hex3, sizeof(hex3));
 	OSMO_ASSERT(rc == -EOVERFLOW);
 
 	/* Data: ext in Octet 5a, but too short */
-	const uint8_t hex4[] = { 0x02, 0x0b, 0x80, 0x80 };
+	const uint8_t hex4[] = { 0x02, 0x0b, 0xc0, 0x80 };
 	rc = gsm0808_dec_channel_type(&ct, hex4, sizeof(hex4));
 	OSMO_ASSERT(rc == -EOVERFLOW);