Wirelessly posted (8800: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0) BlackBerry8310/4.2.2 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102)
The ringtone deal is not an OS issue. The mp3 needs to be under 30 seconds long, and less than 512kb in size. Then set your ringtone to one ring in your profiles. It will play up to 15 seconds which is the carrier default time to voicemail, unless you get that lengthened by calling your carrier.
There is a faq about this, under audio or media, at http://www.blackberryfaq.com/index.php/Main_Page