modem-manager: 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/modem-manager - from package: ModemManager

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

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