Go online to your Google Voice account.
Go to Settings.
On the Phones tab, select "Edit" under your cell phone.
From the Edit screen, select "Show advanced setting"
For the "Voicemail Access" setting select "No (requires pressing star during greetings -- default for non-mobile phones)"
That will then ring all forwarding phones that you have setup.
After the set number of unanswered rings, you will get your own voicemail. From there, you can press the "#" key and enter your PIN to listen to any/all voicemail.