Ngrep is one of networking tools usefull for VoIP (Voice Over IP) analysis. Unfortunatelly I did not find ngrep package available inside ot the yum repository, so command
yum install ngrep
does not work (SipXecs over CentOS). No problem, just few steps how to install it.
Go to your prefered download directory, for example /home/your_username.
Then localize ngrep rpm package available for download, for example at http://packages.sw.be/ngrep/
Download it using wget
When the download is complete, install the rpm by running the following command:
rpm -ivh ngrep-1.45-1.el5.rf.x86_64.rpm
That is all. Try run ngrep command to verify it is working and use it for VoIP analysis as described for example in "Ngrep – a tool for VoIP analysis".