xmonisdn: 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/bin/xmonisdn - from package: xisdnload

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

On Debian possible default full paths are:

/usr/bin/xmonisdn - from package: isdnutils-xtools

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

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