xisdnload: 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/xisdnload - from package: xisdnload

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

On Debian possible default full paths are:

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

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

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