sipwitch: 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/sipwitch - from package: sipwitch

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

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