sipcrack: 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 Debian possible default full paths are:

/usr/bin/sipcrack - from package: sipcrack

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

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