diff --git a/openbsc/src/gsm_04_11.c b/openbsc/src/gsm_04_11.c
index 579bb55..a5f64f1 100644
--- a/openbsc/src/gsm_04_11.c
+++ b/openbsc/src/gsm_04_11.c
@@ -1011,7 +1011,11 @@
 	u_int8_t transaction_id;
 	int rc;
 
-	transaction_id = 4; /* FIXME: we always use 4 for now */
+	transaction_id = trans_assign_trans_id(lchan->subscr, GSM48_PDISC_SMS, 0);
+	if (transaction_id == -1) {
+		DEBUGP(DSMS, "No available transaction ids\n");
+		return -EBUSY;
+	}
 
 	msg->lchan = lchan;