I have FTTP and have recently had this IPC6023 error. After far too many phone calls being passed around different parts of BT , it is clear that this is a known fault with BT and Openreach. Finally after two missed appointments in one week, an Openreach engineer did come and fitted a new C-SERIES MODEM. Now all OK. BT dont seem to work well with Openreach, but you should complain to BT about the whole process - it is a known fault. I did, and was offered compo. I did not complain for the compo - I want a service level to match the £111 I pay them every month. I will give the compo to charity.
