sms2unicode: 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/sms2unicode - from package: smstools

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

Error in other languages:
sms2unicode: bevel nie gevind nie
sms2unicode: comandă negăsită
sms2unicode: comando não encontrado
sms2unicode: commande introuvable
sms2unicode: command not found
sms2unicode: käsku ei ole
sms2unicode: không tìm thấy lệnh
sms2unicode: komanda nerasta
sms2unicode: Kommando nicht gefunden.
sms2unicode: kommandot finns inte
sms2unicode: komut yok
sms2unicode: nie znaleziono polecenia
sms2unicode: níor aimsíodh an t-ordú
sms2unicode: no se encontró la orden
sms2unicode: no s'ha trobat l'ordre
sms2unicode: opdracht niet gevonden
sms2unicode: parancs nem található
sms2unicode: perintah tidak ditemukan
sms2unicode: príkaz nenájdený
sms2unicode: příkaz nenalezen
sms2unicode: команда не найдена
sms2unicode: командата не е открита
sms2unicode: 命令找不到
sms2unicode: コマンドが見つかりません