This refers to a file, which is a common ebook format used primarily by Amazon Kindle devices. mobi file or convert it to a different format?
The code translates to the word "TEXT" using a standard T9 or telephone keypad layout (where 8=T, 3=E, 5=X, 9=T). Therefore, the "proper text" for your string is likely: "text mobi" 8359 mobi