I'm getting a busy signal calling that number (tried both Phonepower and Localphone, twice each). I suspect that you don't get "early media" with GV, so you hear ringback tone instead.
If you have e.g. a Localphone account, you could dial via a calling card access number. (Other calling cards would work, but the 1-855 call might not be free.)
Or, set up a speed dial for SP2(18558439303@sip.tollfreegateway.com) . (Test with a similar speed dial for another toll-free number, so you know it's working. You need SP2 set up for SIP.
Or, set up a VGx for such a service.