callback: 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/sbin/callback - from package: mgetty

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

On Debian possible default full paths are:

/usr/sbin/callback - from package: mgetty

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

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