diff --git a/libpri.h b/libpri.h
index 55009fd..ecfa70c 100644
--- a/libpri.h
+++ b/libpri.h
@@ -573,7 +573,7 @@ int pri_information(struct pri *pri, q931_call *call, char digit);
 
 #define PRI_KEYPAD_FACILITY_TX
 /* Send a keypad facility string of digits */
-int pri_keypad_facility(struct pri *pri, q931_call *call, char *digits);
+int pri_keypad_facility(struct pri *pri, q931_call *call, const char *digits);
 
 /* Answer the incomplete(call without called number) call on the given channel.
    Set non-isdn to non-zero if you are not connecting to ISDN equipment */
diff --git a/pri.c b/pri.c
index afc376a..cb319b7 100644
--- a/pri.c
+++ b/pri.c
@@ -515,7 +515,7 @@ int pri_information(struct pri *pri, q931_call *call, char digit)
 	return q931_information(pri, call, digit);
 }
 
-int pri_keypad_facility(struct pri *pri, q931_call *call, char *digits)
+int pri_keypad_facility(struct pri *pri, q931_call *call, const char *digits)
 {
 	if (!pri || !call || !digits || !digits[0])
 		return -1;
diff --git a/pri_q931.h b/pri_q931.h
index 5f38ee1..0d60a28 100644
--- a/pri_q931.h
+++ b/pri_q931.h
@@ -437,7 +437,7 @@ extern int q931_setup_ack(struct pri *pri, q931_call *call, int channel, int non
 
 extern int q931_information(struct pri *pri, q931_call *call, char digit);
 
-extern int q931_keypad_facility(struct pri *pri, q931_call *call, char *digits);
+extern int q931_keypad_facility(struct pri *pri, q931_call *call, const char *digits);
 
 extern int q931_connect(struct pri *pri, q931_call *call, int channel, int nonisdn);
 
diff --git a/q931.c b/q931.c
index d099438..84052a8 100644
--- a/q931.c
+++ b/q931.c
@@ -2767,7 +2767,7 @@ int q931_information(struct pri *pri, q931_call *c, char digit)
 
 static int keypad_facility_ies[] = { Q931_IE_KEYPAD_FACILITY, -1 };
 
-int q931_keypad_facility(struct pri *pri, q931_call *call, char *digits)
+int q931_keypad_facility(struct pri *pri, q931_call *call, const char *digits)
 {
 	libpri_copy_string(call->keypad_digits, digits, sizeof(call->keypad_digits));
 	return send_message(pri, call, Q931_INFORMATION, keypad_facility_ies);