[Asterisk-Users] How to encript SIP comunications?

[Linux Dominicana]

Hello everybody

 A given scenario:

A client does want to have his own VoIP PBX with Asterisk running, but
he ask me. How secure can be the communication among all subscribers?
If there're sniffers on the middle or any other listening device on a
given netowork.

The client is not fictitial, but it main requirement is encription of
all point to point comunications for given reasons.

Any guidance, products, solutions implementation available and if
works is much better.

Suggestions are welcome

Regards

John Fach