The only way to do so is to answer the call before Google's voicemail does. To guarantee this, have your system answer the call as soon as it arrives. In Asterisk, you will simply call the Answer() function as the first step. If you are using FreePBX, set the context for your trunk to this custom context: 


Add context=custom-gv-answer to your trunk peer definition.


Then enter the following into /etc/asterisk/extensions_custom.conf:


[custom-gv-answer]

exten => _.,1,Answer()

exten => _.,n,Goto(from-pstn,${EXTEN},1)