CSipSimple: Making calls through OpenVPN

The audio cannot be heard because CSipSimple by default advertises the Internet-facing IP address of the phone, not the OpenVPN address. This means the control messages get through (hence one can make calls), but the audio stream is routed out through the Internet and not over the VPN.

Change to Expert mode

To change this, the CSipSimple account "wizard" needs to be changed to "Expert". Assuming you already have the account registered, long-press on the account in the CSipSimple account manager view, and press "Choose Wizard". Then select "Expert" from the "Generic wizards" option.

Change NAT settings

Long-press again on the account name, and select "Modify Account" - there will be many more options than with the basic wizard.

Under the "NAT Traversal" section of the settings, enable "Allow SDP NAT rewrite" - this will tell CSipSimple to use the same address it uses for the control messages and for the audio stream.

Note: this assumes that you are able to place calls, but cannot hear the audio. If you cannot register to Asterisk at all (or whichever SIP server you are using), then it is likely that the VPN is not correctly configured.