isdnrep: 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/isdnrep - from package: isdn4k-utils

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

On Debian possible default full paths are:

/usr/bin/isdnrep - from package: isdnlog

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

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