telnet: 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/lib/heimdal/bin/telnet - from package: heimdal-appl-clients
/usr/kerberos/bin/telnet - from package: krb5-appl-clients
/usr/bin/telnet - from package: telnet

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

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