At least the Nokia 8210 requires hex number to be uppercase.
--- gnokii-0.5.1/common/gsm-encoding.c.orig	2003-06-11 14:32:13.000000000 +0200
+++ gnokii-0.5.1/common/gsm-encoding.c	2003-06-11 14:32:41.000000000 +0200
@@ -313,7 +313,7 @@
 	int i;
 
 	for (i = 0; i < (len / 2); i++) {
-		sprintf(dest + i * 2, "%x", char_def_alphabet_encode(src[i]));
+		sprintf(dest + i * 2, "%X", char_def_alphabet_encode(src[i]));
 	}
 	return;
 }