Thank you for your post.
After one of our Support Technicians downloaded the app and played with a bit, we discovered that using the following command in FileMaker Pro will activate Google Voice:
Open URL ["googlevoice://dial"]
You will need to check with the Google Voice documentation how to pass the login name, password and phone number to dial.
If you find a way to get this working, please feel free to post that information here so others can benefit.