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The Yealink handsets allows you to download a log file which can be quite useful in diagnosing issues. Search for the keyword 'openvpn' in the logs.
Trouble Shooting
Hangs during boot on 'Unable to obtain IP Address'.
If the 'remote' server in you vpn.cnf file contains a FQDN then the phone will need to resolve the FQDN to an IP address before it can start the VPN service.
If it is unable to resolve the FQDN the phone will hang with the above error message.
Resolution:
Change the 'remote' in the vpn.cfn file to be the IP address of the remote.
To stop the phone hanging login to the handsets 'Advanced' settings (not via the web interface as that won't be working).
Select Network and then scroll down to 'VPN'. Disable the VPN and reboot the phone.
You will now be able to upload the new tar file with the 'remote' set to the vpn server's IP address rather than the FQDN.