It is surprising why Google just didn't offer this as a service. You could already build something like this using platforms like Twilio, KOOKOO(in India) etc. We at Ozonetel, already provide a business phone service similar to this for our customers using Google speech as one speech engine, though spam detection is not yet inbuilt. After looking at the launch, I just quickly put together a bot page if someone wants to try it out, https://ering.me/ozonetel/spa/

Note:I am a co founder of KOOKOO.