isdnconf: 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/isdnconf - from package: isdn4k-utils

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

On Debian possible default full paths are:

/usr/bin/isdnconf - from package: isdnlog

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

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