isdnrate: 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/isdnrate - from package: isdn4k-utils

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

On Debian possible default full paths are:

/usr/bin/isdnrate - from package: isdnlog

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

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