icnctrl: 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/icnctrl - from package: isdn4k-utils

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

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