If the phone line is inactive, I pushed mine all back into the cavity and patched the hole. Would never know it was there! - Stuardo
You can get blank plates that you could use from the hardware or electrical shop. We did this and it looked fine and if we ever need to put a powerpoint or data point in - easy to retrofit. - robchin
These wires have a small amount of current running through them so best to leave it intact and push it into the wall. You can also purchase a patch from Bunnings, add some multi purpose compound over a few days with drying and sanding in between, and a lick of paint will bring it back to brand new.
If NBN is connected, your old phone line should be inactive, unless it is connected to the phone point on your provider's modem. Please check if connected, mine was connected to the Optus modem. I disconnected it, because it no longer required the landline. - JoeAzza