imon: 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/sbin/imon - from package: isdn4k-utils

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

On Debian possible default full paths are:

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

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

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