As Heathfield points out, it's not your phone which stops ringing. It's your service provider's system which terminates the call and diverts the caller to their voicemail system.
Hi Mo, just to confirm what heathfield said. I have a Bold and you have to ring your provider and they give you a long string of digits to enter into your phone to lengthen the time.
It does seem daft that you can't do it yourself but it's the only way to do it.