gammu-config: 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/gammu-config - from package: gammu

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

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