telnet-proxy: 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/telnet-proxy - from package: libtelnet-utils

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

On Debian possible default full paths are:

/usr/bin/telnet-proxy - from package: libtelnet-utils

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

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