Here is the very useful trick that saves both your money and time. Even your granny can make or receive calls using an old school phone along with VOIP/SIP supported device. Also, you can configure this in any software that supports basic standard SIP/VOIP.
Note: I DO NOT recommend to use your primary Gmail account as you need to save your Gmail username and password in http://simonics.com. So create a duplicate account just in case...)
1. Create a standard SIP connection to Google Voice using this website (it's free!!) https://simonics.com/gvgw/
Important: If you have never made a call through Google Voice before, you must first log into Gmail chat with your account you just created and initiate a call through the chat widget. Then log into Google Voice, go to your voice settings and select Google Chat as the destination for call forwarding.
2. Fill out the form, double check the details you already entered. Then click the "Add" button.
5. Go to account setup in Empathy and add new SIP account.
Note: If you don't see an option for adding SIP, install this additional plug-in)
User Name: GV#your google voice email@example.com (11 digits without hash)
Password: Secret (Simonics Password)
6. Then select the new SIP account we created and click on "edit." Mark "use this account to call landline and mobile phone."
Settings vary in different platforms.
HELP: Regarding software and devices, if you are stuck anywhere, feel free to ask me or you can also get help from Simonics Blog and Help.
Enjoy free calling and donate some money to Simonics.com and FOSS.