For recording to work, it must be activated in settings. In the mobile app or or via voice.google.com on the desktop, go to Settings > Calls > Incoming call options.
The effing spammers are using this frequently. I wish Sprint would turn this off. Their G.D. spam goes directly to my mailbox without me being able to kill the call first.

2. Enter the destination you wish to transfer the call to, then press [SEND] softkey. 3. Once the receiving party answers, announce the call. 4. To complete the transfer, press again. If you hang up before the call rings, the transfer will fail and the call will be disconnected. Direct to Voicemail: Transfer a call directly to voicemail 1. Press . 2. If a phone rings once and then goes to voicemail or rings only briefly, it You can't definitively know the cause of a phone barely ringing and going to voicemail because neither a phone company nor the phone's operating system provider will tell you if your phone number is blocked.
Some networks charge a flat monthly fee to use voicemail, while others charge for each individual voicemail message, or charge by the minute while you're connected to the voicemail system. Some networks even charge you for simply recording a voicemail message - even if you never bother listening to the message.