I had the same problem for quite a while. Redialing a few times would sometimes work. However, if I used the 1-800 access number, it would work the first time. Dialing using Reliance app on Android phone also worked without any problems. Must have been some issue with the interface from the Obi110 through GV.
I am not seeing this problem since last weekend on the phone connected to my Obi110 using Google Voice. Hope this problem has been taken care of. Check and see if it is working for you too.