As it's been noted by some previous users, Ooma apparently starts counting call duration as soon as the caller dials a number and/or the phone starts ringing - not when the person at the other end actually picks up the phone and starts talking. This appears to be the reason why my failed calls are responsible for about 50 minutes out of my 600 minutes total.
My rating of Ooma service on a scale of 1 to 5 stars (where 5 is best, 1 is worst):
International Call Quality: 3.5 stars out of 5.0 ***
Domestic US Call Quality: 4.5 stars out of 5.0 ****
Type of Internet (Broad-band / Dial-up / DSL): Broad-band
Additional: Ooma customer service appears to be quite satisfactory.
My wife calls China using the same plan you are using with great success.
I found that the following settings have helped with our International Calling Experience.
To stabilize your Ooma device connection change from Automatic to Use Built In MAC Address:
If your Ooma setup is Modem-Ooma-Router, type http://172.27.35.1 in your computer's browser. The Ooma Setup pages opens. Click on Internet settings. Go down to INTERNET port MAC Address: and click on Use Built In. Click on Update device and restart your Ooma device.
If your Ooma device is connected behind your router, temporarily connect a network cable from one of your router's LAN ports to the wired LAN port of a computer. Temporarily disable Wi-Fi. Reboot the computer.
Than type http://172.27.35.1 in your computer's browser and select Use Built MAC address per instructions above. Reboot your Ooma Device.
If your Ooma device is connected Modem-Ooma-Router access your Ooma Setup pages and click on Advanced. Set Quality of Service Upstream and Downstream values to about 80 % of measured Upload and download speeds as measured by http://www.speedtest.net/. Note: 80 % is just a starting place and this value may have to raised or lowered slightly as required for your home's LAN requirements. Reboot your Ooma Device.
If your Ooma device is connected Modem-Router-Ooma set the Ooma Setup Advanced Quality of Service Upstream and Downstream values to zero. Reboot your Ooma Device.
For Modem-Router-Ooma connection Only: If your Ooma device is connected Modem-Router-Ooma reserve a static IP address in your router for your Ooma device. Than place the Ooma device's static IP address in your router's DMZ. Reboot your Router and Reboot your Ooma Device.
For Modem-Router-Ooma connection Only: If you still have problems with dropped calls, take the router static IP address information from your router and add it to your Ooma device, by accessing Ooma Setup and clicking on Internet Settings. Click on Connection Type down arrow and select Static IP address. Under Static IP Address Options, populate the fields as follows:
IP address: is the IP address you reserved in your router for the Ooma device.
Netmask: is usually 255.255.255.0, but double check in your LAN settings in your router.
DNS Server 1 & Router Address is the same as your Ooma device's static IP address, except with a 1 at the end. Example xxx.xxx.xxx.1
DMS Server 2: requires nothing.
Reboot your Ooma Device.
To give an idea of how your Internet connection is doing, do the following:
Run http://speedtest.phonepower.com/. Click on the Advanced tab, than "view text" in the lower right had corner of the page. Highlight the text, copy data and paste data here in a new post.
Your Router IP Address is used for both the DNS Server 1 and Router Address. The Router IP Address is 192.168.1.1, unless you have changed them from factory settings.