I do not believe this feature can be modified, actually I believe it may be the ringer you are using because the system is configured to ring 4 times before it forwards to voicemail, the ringer you are using may be to long and this may be why you only hear it ring twice, have you tried calling your phone from another phone and see exactly how many rings the caller recieves before being tranferred. If it rings more than two times, I suggest to either shorten ringer or try another one.
I use my mp3 songs as ringers... and they seem to ring about 20 - 30 seconds..
I use mp3's as ringers also. I used a lot of beach boys, Frank Sinatra, Bing and Dino, and when I made them, I made them 30 seconds long because that's how long it rang before going to voicemail.
But to answer your question.... No, I don't think you can extend the ring time before going to voicemail. It's set at approx. 30 seconds