Some phones like the ATCOM 530 now support ipsec to establish a VPN. Elastix comes now with the ipsec-tools package to make it possible to create a tunnel between the Elastix PBX and some phones.

This is a good reading:

http://www.centos.org/docs/5/html/Deployment_Guide-en-US/ch-vpn.html

Note that at this moment we are not mentioning support for the SRTP protocol or “Secure RTP”.

If you are interested in SRTP please take a look at this thread http://bugs.digium.com/view.php?id=5413