I asked a question about the voice messages the other day, but I don't think anyone ever considered it.
For the spoofing to work, would it require a "clean" line? By clean, I mean one that goes straight to dial tone? Would a phone line that has the voice message "signal" (the beeps or whatever that most lines have if a voice message is waiting) interrupt a spoofing or transferring call?
Most of the methods they described would not. Cool thought.