These providers appear to be supported:
    Verizon Wireless
    AT&T
    Sprint (As well as Boost Mobile, Virgin Mobile, Nextel and other related providers)
    T-Mobile
    Tracfone (And other related providers)
  These providers appear to be unsupported:
    Cricket Wireless
    US Cellular

according to http://www.google.com/support/forum/p/voice/thread?tid=76caf...