- Receive the call on your Google Voice number. While talking, press 4 on your phone handset. You can press 4 at the time of taking the call, or at any time during the call.
- Listen to the automated "Call recording on" which will play for all parties on the phone. Continue talking as your call is recorded.
- Press 4 again at any time you wish to end the recording. Hanging up will also have the same effect.
Source : WikiHow