modemtest: 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/modemtest - from package: perl-Device-SerialPort

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

On Debian possible default full paths are:

/usr/bin/modemtest - from package: libdevice-serialport-perl

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

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