If you want the VM indicators to light on all phones then you will have to datafill then all with the same extension number. They will all ring at the same time and the VM indicator will flash on all sets. Access to VM will be identical on all sets. If you need the VM indicator on only a few but you still want all sets to ring, the create a ring group and add all the extensions. Give only a few sets the same number, those will have VM indication and access but all phones in the ring group will ring, if you set the RG option that way. Don't forget to set how the PBX sees the busy state of the extensions or you won't get a second call if you have all sets with the same number.