If you are using your Google Voice for another purpose, such as a SIP service for an ObiHai appliance or as a VOIP system on your PC, you will encounter problems with Voicemail not picking up. You will need to disable Google Voicemail on your phone, create a new Google Voice account, with a new number, and add your Android phone to that account. Note: your Android mobile phone can be attached to more than one Google Voice account! Then select "use Google voicemail" and follow the instructions for your provider.
(If you use Straight Talk, select either T-Mobile or AT&T as your provider, depending on the SIM you purchased from Straight Talk.)
The number on the new Google Voice account does not need to be used for anything other than voicemail - calls to your cell phone will be forwarded to the GV number after 4 rings, and since there is no VOIP device listening at that number on the new account, voicemail will pick up.
You will need to log into the new account in the Google Voice app, and go through the setup wizard again.