sipp: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/sipp - from package: sipp

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/sipp - from package: sip-tester

You may install the required package with command apt-get install {package}

Error in other languages:
sipp: bevel nie gevind nie
sipp: comandă negăsită
sipp: comando não encontrado
sipp: commande introuvable
sipp: command not found
sipp: käsku ei ole
sipp: không tìm thấy lệnh
sipp: komanda nerasta
sipp: Kommando nicht gefunden.
sipp: kommandot finns inte
sipp: komut yok
sipp: nie znaleziono polecenia
sipp: níor aimsíodh an t-ordú
sipp: no se encontró la orden
sipp: no s'ha trobat l'ordre
sipp: opdracht niet gevonden
sipp: parancs nem található
sipp: perintah tidak ditemukan
sipp: príkaz nenájdený
sipp: příkaz nenalezen
sipp: команда не найдена
sipp: командата не е открита
sipp: 命令找不到
sipp: コマンドが見つかりません