I've had it too. Contact your service provider and make sure that all calls are forwarded to your GV number and you'll be good to go.

 

Try cycling your Service for Voicemail in the settings. This may reset the call forwarding for you and update your service provider. Verify that that Setup (GV phone number and account) are correct at the same time, same settings section.

Additionally you can set the call forwarding yourself in the Call Forwarding section of the settings. You just have to manually put in yourself in the selection box (you can choose from the Contact list).

 

Not all carriers support what's called conditional forwarding. TMobile's prepaid plans, to my understanding, do not support this, but the postpay plans do. So you may need to check with your carrier to ensure it's supported and setup right.