I have been using nettalk duo for a few weeks, I am posting my personal experience here which someone may find it heplful.
Good thing:
1. good voice quality.
2. support domestic and international fax.
bad thing:
1. It took 24 hrs for my nettalk duo device to register to the server successfully.
2. You will be charged around 2% of amount if you buy credit by using credit card or paypal.
I have another another question, is it possible that two voip adaptor are connected to one router? for example, I already have nettalk duo connected to the router, can I connect Obi110 to the same router so I can have two voip lines?
Thanks