tel: command not found

Have You tried accessing the binary with the full path?

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 Debian possible default full paths are:

/usr/bin/tel - from package: orville-write

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