diff --git a/include/osmocom/gsm/gsm23003.h b/include/osmocom/gsm/gsm23003.h
index 2bc7583..dd41bc5 100644
--- a/include/osmocom/gsm/gsm23003.h
+++ b/include/osmocom/gsm/gsm23003.h
@@ -3,6 +3,7 @@
 #pragma once
 
 #include <stdint.h>
+#include <stdbool.h>
 
 /* 23.003 Chapter 12.1 */
 struct osmo_plmn_id {
@@ -81,3 +82,6 @@
 	struct osmo_gummei gummei;
 	uint32_t mtmsi;
 };
+
+bool osmo_imsi_str_valid(const char *imsi);
+bool osmo_msisdn_str_valid(const char *msisdn);
diff --git a/include/osmocom/gsm/protocol/gsm_23_003.h b/include/osmocom/gsm/protocol/gsm_23_003.h
index 94243fe..0e66939 100644
--- a/include/osmocom/gsm/protocol/gsm_23_003.h
+++ b/include/osmocom/gsm/protocol/gsm_23_003.h
@@ -4,6 +4,7 @@
 
 /* Chapter 2.2 */
 #define GSM23003_IMSI_MAX_DIGITS	15
+#define GSM23003_IMSI_MIN_DIGITS	6
 /* Chapter 2.4 */
 #define GSM23003_TMSI_NUM_BYTES		4
 /* Chapter 2.5 */