Google Voice app now supports iPad & iPod Touch - Official Google Mobile Blog

I.e., it’s the voice app with which you can’t directly initiate voice communication (except as a remote control for a separate phone)

At the top of the list was adding support for iPod Touch and iPad. Starting today, you can download a new version of Google Voice that lets you use all the features of the app on these devices, such as sending and receiving free text messages - except, of course, make cellular calls.
While you can’t use your iPod or iPad as a phone, you can use it to initiate Google Voice calls with your phones. We call this feature Click2Call. Simply click any ‘Call’ button in the Google Voice app on your iPod or iPad and then select which of your phones you want to ring. Google Voice will call your phone and then connect your call.

Google Voice app now supports iPad & iPod Touch - Official Google Mobile Blog