Have been on NBN for some months, with optus sagemcom 3864V3 modem. Have phone and two computers connected: one hardwired, the other connected via wifi. System worked fine. Then phone and internet went down. After investigating, it looked like modem had intermittent fault. Optus sent me a new one, but problems persisted. After further investigation, it appeared that although the wifi doesn't always go down, the phone and harwired computer invariably fail sometime after my hardwired computer is turned on (typically after 1-2 hours). But how can my computer affect the phone which is connected to the modem? Any ideas on how to fix this?
Your phone is using a protocol called Voice over IP, on NBN. So if your Internet Connections drops out, your phone will not work, as you voice is being tunneled over the Internet Connection back to Optus. It sounds very much you are having drop outs on your NBN Service. So you know what technology you are connected on, is it Fibre to the Node? If so, I suggest you unplug all the other devices from your sockets apart from the modem, or you may need to get a licenced cable to unhook the sockets.
Have you tried contacting Technical Support to see if they can see the drop outs on NBN Side? You may need the stability profile added to your line (will slow your connection down), or you may have a fault.