It seems like a CSS/parition issue on CCM. If you have a sepeate Parition for VM hunt pilot make sure this partiion is listed ahead of the voicemail ports parition in the Gateway Calling Search Space.
I guess whats happening is your voicemail hunt pilot has the same DN as the first voicemail port, but the GW CSS lists the voicemail ports parition first so it's sending the call to this voicemail port instead of the hunt pilot, so when second call arrives it gets busy tone.
HTH,
Chris