I've just completed set up of GV as voicemail for a client's new Nexus 5, I had a few issues with busy/no answers being told something along the lines as "There was a problem, please try again later." until I realized 'Voicemail' had to be enabled on the dashboard. What follows are instructions on how I was able to set up GV as Voicemail for this client.
Here it goes.
On the Web
Get a GV# from voice.google.com
On the phone
Install GV app "Voice"
Dial *72(GV#) (ex. *721112223333)
Update GV number in GV app
On the Ting Dashboard:
Make sure "Voicemail" is checkmarked.
Checkmark Forward Busy signal to this # (type GV#)
Checkmark Forward "No Answer" calls to this # (type GV#)
This configuration method works on the three phones I've set up for them thus far.