Install OpenSIPS SIP server

What OpenSIPS is

OpenSIPS (Open SIP Server) is a mature Open Source implementation of a SIP server. OpenSIPS is more than a SIP proxy/router as it includes application-level functionalities. OpenSIPS, as a SIP server, is the core component of any SIP-based VoIP solution. With a very flexible and customizable routing engine, OpenSIPS 'unifies voice, video, IM and presence services in a highly efficient way, thanks to its scalable (modular) design.

What OpenSIPS has to offer, comes in a reliable and high-performance flavour - OpenSIPS is one of the fastest SIP servers, with a throughput that confirms it as a solution up to enterprise or carrier-grade class.

Adding Debian package repository for the OpenSIPS project to your apt sources:
# vi /etc/apt/source.list
deb http://apt.opensips.org/ stable18 main

Adding the archive key to your keyring:
This archive self-signs packages uploaded to it (and packages uploaded are verified against a whitelist of trusted uploaders) using OpenPGP (GnuPG, to be more precise).

The key of the archive is in the key.asc file. You should add the key using something like this:
# wget http://apt.opensips.org/key.asc
# apt-key add key.asc
# apt-get update

# apt-get instal opensips

Reference:
http://www.opensips.org/
http://apt.opensips.org/