isdnctrl: 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:

/sbin/isdnctrl - from package: isdn4k-utils

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

On Debian possible default full paths are:

/usr/sbin/isdnctrl - from package: isdnutils-base

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

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