[Paul K]

If you were in the main socket (test socekt is preferable) and had done the checks given to you then there would be no risk of a charge. It is up to the support person to get you through the checks correctly and ensure that the engineer goes out if needed. These issues can sometimes be down to:
Faulty filter
faulty cable from filter to router/ modem
faulty socket
faulty line equipment between socket and exchange