sip-dig: command not found

Have You tried accessing the binary with the full path?

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/sip-dig - from package: sofia-sip-bin

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