T21P : no audio when connect via openvpn

[kohan]

Thank you for your fast respon good advice.. it's very useful..
i had try your advice, my openvpn is working and i can talk with yealink in remote site with that openvpn system..
but now i have problem again..
When yealink connected to voip server via openvpn server and the set time/clock is "DHCP time enable", the yealink time/clock change to default setting (fabrication time maybe, january 2015), or with another word my yealink is not receives time/clock setting from openvpn server.. with that time/clock setting, yealink can not connect with openvpn server because CA/client certificate expired (certificate validity from April 2016 - 2026)
when yealink setting is "DHCP time enable", i can not change "manual time" from disable to enable..
when i change yealink setting from DHCP time" from enable to enable and i set "manual time" enable and i set with real time/clock, yealink can connect with voip server via openvpn server, but if I restart yealink phone, time/clock setting change to "DHCP time enable" in january 2015 and yealink can't connect to openvpn server again..

What do you think about my case/problem? Is the problem in my openvpn server or in yealink phone?
Can you help me solve my problem?

Thank you..