I have the same problem. For me it only seems to happen when I talk about things I'm passionate about, such as work. This is way I avoid the phone at all times. I have never met anyone who cared (or even mentioned it to me other than my wife). The problem with phones (for me) is that people can't see you. My stutter involves pauses which in a face to face conversation are pretty obvious that I'm right in the middle of trying to say something. On the phone these pauses are weird and when it happens it's not uncommon to hear on the other end; "Are you still there..?".

I can imagine that speaking to investors would be a major problem. Not so much that anyone would can about your stutter or stammer, but more about the frustration with yourself that you can't properly communicate your idea. You always find yourself having to compromise on what it is you're trying to bring across.

Unfortunately I have no tips or suggestions since I haven't been able to find any myself.