Is there a Google Voice API?

There is a C# Google Voice API... there is limited documentation, however the download has an application that 'works' using the API that is included:

https://sourceforge.net/projects/gvoicedotnet/

No, there is no API for Google Voice announced as of 2021.

"pygooglevoice" can perform most of the voice functions from Python. It can send SMS. I've developed code to receive SMS messages, but the overhead is excessive given the current Google Voice interface. Each poll returns over 100K of content, so you'd use a quarter-gigabyte a day just polling every 30 seconds. There's a discussion on Google Code about this.