This week's Ask Engadget comes to us from Engadget pal Nick B., who wants to know, "What's the most useful portable Voice over IP service?" So what is it? A WiFi Pocket PC with loaded up with Skype or Vonage? A laptop with a softphone client? Just carry around your Analogue Telephone Adapter and hope for the best? Anyone tried one of those newfangled WiFi phones, like the ZyXEL Prestige 2000W pictured at right?