dahdi_monitor: 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/dahdi_monitor - from package: dahdi-tools

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

On Debian possible default full paths are:

/usr/sbin/dahdi_monitor - from package: dahdi

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

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