I have an Obihai OBi200 1-Port VoIP Phone Adapter. For many months, it is been working fine. Today it will provide a dialtone and you can dial a number (and I have tried many). It rings then it will then give you a message that it can't connect. However, using the Google voice app on my win 10 laptop, I can connect to those same numbers without a flaw.
I have rebooted it several times and it gives the same result.