In pidgin there is a plugin that we can use to encrypt our messages in chat windows .. Is there anything simular for voip (pc to pc) ? any client that support manual encryption for voice data?(linux)
If you can directly connect, you can tunnel just about anything over SSL using
stunnel. Any other sort of port-forwarding arrangement would also work. If you're using something more complicated involving SIP, you'll have to build a custom application or a fancier proxy that works in tandem with directory services.
I haven't used this in forever, nor has it been updated in a decade, but Speak Freely was a cool tool in its day that included crypto that the end user was responsible for. It's under the GPL.
Since you're mentioning Linux, if both PC's run Linux you can set up "IPSec transport" between the two systems and you'll get your manually keyed, encrypted VoIP sessions using whatever VoIP client you want!
You can also use a VPN, and I recommend using OpenVPN. VoIP works best over UDP, and OpenVPN itself uses UDP, so it can be called an UDP-friendly VPN. That said, I've used VoIP over PPTP (the kind of VPN you get for free on a Windows computer). I've mostly done that to circumvent poor routers in hotels - but I got encrypted phone conversations as a side effect. I used PPTP because it's readily available on my Android phone.
HD Audio and video call
Secure communications (TLS, SRTP, zRTP)
For detailed instructions on how to set up secure calls with Linphone see: