An interesting question. I've seen several different types of behavior. Right now, I still have one SP on GV which I will not move until it formally ends, since the voice quality is excellent as I see it, and the convenience and reliability are not anything I have to think about.
So I just tried to dial my own GV number from the GV/Gtalk phone line, and my Google voice mail came on immediately. So it "knows" what phone you are calling from, and it sends you to your own voice mail.
I don't know how this works with other SIP lines, but when I had a land line, it worked the same way when calling the GV number from a land line listed as a forwarding number.
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I do not currently have an outgoing provider on my SIP line, other than GV. So I just initiated a callback from my GV page of my SIP line to my own GV DID number (which is the only way I have right now to place an outgoing call from that line). My phone rang as normal and when I picked up, the same GV voice mail attendant came on "you have NO new messages. To place a call.... "
So my evidence suggests that it is SUPPOSED to work just like standard cell phone/phone co voice mail. You call your own number, and it kicks you into voice mail, just like an outside caller calling into a busy line. So I suspect your vestalink is not functioning right. Or at least not acting the way it is designed / intended to.