that won't make any difference... just extend the number of vibrations and shorten the actual tone part of the ring.
I wanted to do this as well and I called T-Mobile on it a couple of months ago. They set something up on my phone with No Answer -Call Forwarding so that it actually CFs to my same number giving me twice the ring time before going to VM. I'm not sure how it works because I figured that would cause an endless loop of ringing until the caller hung up and it would never reach my VM.... but it does!
Hopefully the rep you talk to if you call knows what to do. It was a sequence of numbers I had to push into my phone...