Recently got a new phone, and it took me awhile to figure out how to get all my unanswered calls forwarded to Google Voice automatically.
My carrier, T-Mobile, has a list of short codes that can be used to set this stuff up, but the instructions there leave a lot to be desired.
I had to use the following set of short codes to get the call forwarding to work correctly:
Forward calls to Google Voice number when the line is not answered
**62*[Phone number starting with country code 1]#Forward calls to Google Voice number when the line is in use
**67*[Phone number starting with country code 1]#Set length of time before forwarding calls to voice mail
**61*18056377243**[seconds - 5, 10, 15 or 30]#