modem-cmd: 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 Debian possible default full paths are:

/usr/bin/modem-cmd - from package: modem-cmd

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

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