isdnlog: 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/isdnlog - from package: isdn4k-utils

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

On Debian possible default full paths are:

/usr/sbin/isdnlog - from package: isdnlog

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

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