RTTTL is somewhat uncommon audio format and unfortunately not many programs support it. Nevertheless, some dedicated audio editing software, like for example the Awave Studio supports it, and allow users to convert common MP3 files to RTTTL files so they can be used as ringtones in very old Nokia phones. This is pretty much the only reason why mp3 to rtttl conversion is done anyway.
Open MP3 file Open RTTTL file