diff --git a/include/libtcap.h b/include/libtcap.h
index da919da..1ab435b 100644
--- a/include/libtcap.h
+++ b/include/libtcap.h
@@ -26,6 +26,7 @@ void *inap_decode(Invoke_t *invoke, asn_TYPE_descriptor_t **);
 int tcap_extract(const char *buf, size_t len, const char *spec, struct output_buffer *out);
 int inap_extract(const char *buf, size_t len, const char *spec, struct output_buffer *out);
 int isup_convert_number(const char *inp, int inlen, char *out);
+int isup_convert_number_hex(const char *inp, int inlen, char *out);
 
 
 #endif
diff --git a/src/tcap.c b/src/tcap.c
index 59d0f0b..5b32c5a 100644
--- a/src/tcap.c
+++ b/src/tcap.c
@@ -298,18 +298,9 @@ error:
 
 
 static inline char phone_digit(unsigned char d) {
+	if (d >= 0 && d <= 9)
+		return '0' + d;
 	switch (d) {
-		case 0:
-		case 1:
-		case 2:
-		case 3:
-		case 4:
-		case 5:
-		case 6:
-		case 7:
-		case 8:
-		case 9:
-			return '0' + d;
 		case 11:
 			return '*';
 		case 12:
@@ -321,8 +312,18 @@ static inline char phone_digit(unsigned char d) {
 	}
 }
 
+static inline char phone_digit_hex(unsigned char d) {
+	if (d >= 0 && d <= 9)
+		return '0' + d;
+	else if (d >= 10 && d <= 15)
+		return 'A' + d - 10;
+	return '?';
+}
 
-int isup_convert_number(const char *inp, int inlen, char *out) {
+
+static inline int isup_convert_number_common(const char *inp, int inlen, char *out,
+		char (*func)(unsigned char))
+{
 	const char *pi;
 	unsigned char raw[32] = "", *pr;
 	int rawlen, odd;
@@ -380,8 +381,8 @@ int isup_convert_number(const char *inp, int inlen, char *out) {
 	pr = raw + 2;
 	rawlen -= 2;
 	while (rawlen) {
-		*(out++) = phone_digit(*pr & 0xf);
-		*(out++) = phone_digit(*pr >> 4);
+		*(out++) = func(*pr & 0xf);
+		*(out++) = func(*pr >> 4);
 		rawlen--;
 		pr++;
 	}
@@ -396,3 +397,10 @@ int isup_convert_number(const char *inp, int inlen, char *out) {
 error:
 	return -1;
 }
+
+int isup_convert_number(const char *inp, int inlen, char *out) {
+	return isup_convert_number_common(inp, inlen, out, phone_digit);
+}
+int isup_convert_number_hex(const char *inp, int inlen, char *out) {
+	return isup_convert_number_common(inp, inlen, out, phone_digit_hex);
+}