isdnconfig: command not found

Have You tried accessing the binary with the full path?

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 Debian possible default full paths are:

/usr/sbin/isdnconfig - from package: isdnutils-base

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