vboxctrl: 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/vboxctrl - from package: isdn4k-utils

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

On Debian possible default full paths are:

/usr/bin/vboxctrl - from package: isdnvboxclient

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

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