Check if this is for both BT ISDN and VoIP calls. Is the message only when dialling 9 for an outside line? Can VoIP calls be made to other stores? Dial 6 then dial the store number followed by the extension number. If a VoIP call can be made then its probably a BT ISDN fault. Raise a ticket.
Check the lights on the ISDN boxes, are they solid, flashing or not on?
Solid – Lines should be ok. Report to Quexia.
Flashing – Possible system error but if VoIP works then this could still be a BT issue. Report to Quexia.
Not Illuminated – BT ISDN line fault. Report to Quexia. Note, no BT weekend cover.