After the announcement that Google Voice would stop supporting SIP and VoIP phone adapters as of May 2014, I switched my Google Voice phone number to VoIP.MS
That all went well, as per a previous post here.
Now, Google Voice have changed their stance, and now support the full range of ATA (analog telephone adapter) adapters from Obihai. See this post:
I currently use two Obihai ATA adapters and recommend them. I have never had any problems setting them up, and taking one of them with me when travelling for extended periods.