isdnbill: 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/isdnbill - from package: isdn4k-utils

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

On Debian possible default full paths are:

/usr/bin/isdnbill - from package: isdnlog

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

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