diff --git a/include/osmocom/hlr/gsup_server.h b/include/osmocom/hlr/gsup_server.h
index 9c562e22..86ae4ec3 100644
--- a/include/osmocom/hlr/gsup_server.h
+++ b/include/osmocom/hlr/gsup_server.h
@@ -69,8 +69,6 @@ void osmo_gsup_server_destroy(struct osmo_gsup_server *gsups);
 int osmo_gsup_configure_wildcard_apn(struct osmo_gsup_message *gsup,
 				     uint8_t *apn_buf, size_t apn_buf_size);
 int osmo_gsup_create_insert_subscriber_data_msg(struct osmo_gsup_message *gsup, const char *imsi, const char *msisdn,
-					    uint8_t *msisdn_enc, size_t msisdn_enc_size,
-				            uint8_t *apn_buf, size_t apn_buf_size,
-					    enum osmo_gsup_cn_domain cn_domain);
+					    enum osmo_gsup_cn_domain cn_domain, void *talloc_ctx);
 int osmo_gsup_forward_to_local_peer(struct osmo_gsup_server *server, const struct osmo_cni_peer_id *to_peer,
 				    struct osmo_gsup_req *req, struct osmo_gsup_message *modified_gsup);
diff --git a/src/gsup_server.c b/src/gsup_server.c
index f0d8101f..097a854d 100644
--- a/src/gsup_server.c
+++ b/src/gsup_server.c
@@ -446,19 +446,16 @@ int osmo_gsup_configure_wildcard_apn(struct osmo_gsup_message *gsup,
  * \param[out] gsup  The gsup message to populate.
  * \param[in] imsi  The subscriber's IMSI.
  * \param[in] msisdn The subscriber's MSISDN.
- * \param[out] msisdn_enc A buffer large enough to store the MSISDN in encoded form.
- * \param[in] msisdn_enc_size Size of the buffer (must be >= OSMO_GSUP_MAX_CALLED_PARTY_BCD_LEN).
- * \param[out] apn_buf A buffer large enough to store an APN (required if cn_domain is OSMO_GSUP_CN_DOMAIN_PS).
- * \param[in] apn_buf_size Size of APN buffer (must be >= APN_MAXLEN).
  * \param[in] cn_domain The CN Domain of the subscriber connection.
+ * \param[in] talloc_ctx To allocation memory for dynamic fields (msisdn, apn) in the gsup field
  * \returns 0 on success, and negative on error.
  */
 int osmo_gsup_create_insert_subscriber_data_msg(struct osmo_gsup_message *gsup, const char *imsi, const char *msisdn,
-						uint8_t *msisdn_enc, size_t msisdn_enc_size,
-						uint8_t *apn_buf, size_t apn_buf_size,
-						enum osmo_gsup_cn_domain cn_domain)
+						enum osmo_gsup_cn_domain cn_domain,
+						void *talloc_ctx)
 {
 	int len;
+	uint8_t *msisdn_buf = talloc_size(talloc_ctx, OSMO_GSUP_MAX_CALLED_PARTY_BCD_LEN);
 
 	OSMO_ASSERT(gsup);
 	*gsup = (struct osmo_gsup_message){
@@ -467,27 +464,22 @@ int osmo_gsup_create_insert_subscriber_data_msg(struct osmo_gsup_message *gsup,
 
 	osmo_strlcpy(gsup->imsi, imsi, sizeof(gsup->imsi));
 
-	if (msisdn_enc_size < OSMO_GSUP_MAX_CALLED_PARTY_BCD_LEN)
-		return -ENOSPC;
-
-	OSMO_ASSERT(msisdn_enc);
-	len = gsm48_encode_bcd_number(msisdn_enc, msisdn_enc_size, 0, msisdn);
+	len = gsm48_encode_bcd_number(msisdn_buf, OSMO_GSUP_MAX_CALLED_PARTY_BCD_LEN, 0, msisdn);
 	if (len < 1) {
 		LOGP(DLGSUP, LOGL_ERROR, "%s: Error: cannot encode MSISDN '%s'\n", imsi, msisdn);
 		return -ENOSPC;
 	}
-	gsup->msisdn_enc = msisdn_enc;
+	gsup->msisdn_enc = msisdn_buf;
 	gsup->msisdn_enc_len = len;
 
 	#pragma message "FIXME: deal with encoding the following data: gsup.hlr_enc"
 
 	gsup->cn_domain = cn_domain;
 	if (gsup->cn_domain == OSMO_GSUP_CN_DOMAIN_PS) {
-		OSMO_ASSERT(apn_buf_size >= APN_MAXLEN);
-		OSMO_ASSERT(apn_buf);
+		uint8_t *apn_buf = talloc_size(talloc_ctx, APN_MAXLEN);
 		/* FIXME: PDP infos - use more fine-grained access control
 		   instead of wildcard APN */
-		osmo_gsup_configure_wildcard_apn(gsup, apn_buf, apn_buf_size);
+		osmo_gsup_configure_wildcard_apn(gsup, apn_buf, APN_MAXLEN);
 	}
 
 	return 0;
diff --git a/src/hlr.c b/src/hlr.c
index 8183d9b9..e3b54acd 100644
--- a/src/hlr.c
+++ b/src/hlr.c
@@ -85,8 +85,6 @@ osmo_hlr_subscriber_update_notify(struct hlr_subscriber *subscr)
 
 	llist_for_each_entry(co, &g_hlr->gs->clients, list) {
 		struct osmo_gsup_message gsup = { };
-		uint8_t msisdn_enc[OSMO_GSUP_MAX_CALLED_PARTY_BCD_LEN];
-		uint8_t apn[APN_MAXLEN];
 		struct msgb *msg_out;
 		uint8_t *peer;
 		int peer_len;
@@ -131,8 +129,7 @@ osmo_hlr_subscriber_update_notify(struct hlr_subscriber *subscr)
 		     subscr->imsi, cn_domain == OSMO_GSUP_CN_DOMAIN_PS ? "PS" : "CS",
 		     osmo_quote_str(peer_compare, -1));
 
-		if (osmo_gsup_create_insert_subscriber_data_msg(&gsup, subscr->imsi, subscr->msisdn, msisdn_enc,
-								sizeof(msisdn_enc), apn, sizeof(apn), cn_domain) != 0) {
+		if (osmo_gsup_create_insert_subscriber_data_msg(&gsup, subscr->imsi, subscr->msisdn, cn_domain, OTC_SELECT) != 0) {
 			LOGP(DLGSUP, LOGL_ERROR,
 			       "IMSI='%s': Cannot notify GSUP client; could not create gsup message "
 			       "for %s:%u\n", subscr->imsi,
diff --git a/src/lu_fsm.c b/src/lu_fsm.c
index deb180bd..e6ed31af 100644
--- a/src/lu_fsm.c
+++ b/src/lu_fsm.c
@@ -241,13 +241,11 @@ static void lu_fsm_wait_insert_data_result_onenter(struct osmo_fsm_inst *fi, uin
 	struct lu *lu = fi->priv;
 	struct hlr_subscriber *subscr = &lu->subscr;
 	struct osmo_gsup_message gsup;
-	uint8_t msisdn_enc[OSMO_GSUP_MAX_CALLED_PARTY_BCD_LEN];
-	uint8_t apn[APN_MAXLEN];
 
 	if (osmo_gsup_create_insert_subscriber_data_msg(&gsup, subscr->imsi,
-							subscr->msisdn, msisdn_enc, sizeof(msisdn_enc),
-							apn, sizeof(apn),
-							lu->is_ps? OSMO_GSUP_CN_DOMAIN_PS : OSMO_GSUP_CN_DOMAIN_CS)) {
+							subscr->msisdn,
+							lu->is_ps ? OSMO_GSUP_CN_DOMAIN_PS : OSMO_GSUP_CN_DOMAIN_CS,
+							OTC_SELECT)) {
 		lu_failure(lu, GMM_CAUSE_NET_FAIL, "cannot encode Insert Subscriber Data message");
 		return;
 	}