Assuming these phones are forwarding phones, when you call your GV number from forwarding phones, the default setting is to go directly to GV Voicemail. This is about to change 10/1/13, see this:
https://support.google.com/voice/answer/117523?p=presecurevoicemail&rd=1There is a GV Settings that directly controls, look under Settings > Phones > Click a Forwarding Phone > Advanced Settings > Voicemail Access: Direct access to voicemail when calling your Google number from this phone? It sounds like you have this set to yes.