According to a video at www.cisco.com, the 7921G supports 6 line appearances.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@104920 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_skinny.c

@@ -1176,11 +1176,14 @@ static void *get_button_template(struct skinnysession *s, struct button_definiti
 			(btn++)->buttonDefinition = BT_HOLD;
 			break;
 		case SKINNY_DEVICE_7920:
-		case SKINNY_DEVICE_7921:
 			/* XXX I don't know if this is right. */
 			for (i = 0; i < 4; i++)
 				(btn++)->buttonDefinition = BT_CUST_LINESPEEDDIAL;
 			break;
+		case SKINNY_DEVICE_7921:
+			for (i = 0; i < 6; i++)
+				(btn++)->buttonDefinition = BT_CUST_LINESPEEDDIAL;
+			break;
 		case SKINNY_DEVICE_7902:
 			ast_log(LOG_WARNING, "Unsupported device type '%d (7902)' found.\n", d->type);
 			break;