No, you shouldn't need a filter for NBN FTTN.
Most phones should be compatible with being connected to the voice port on the T-Gateway (or Gateway Max), it doesn't have to be a DECT phone, but DECT phones do allow you to connect it wirelessly a little easier.
Are you just not having any dial tone or any response or is there something specifically that isn't happening?
Are you able to access the control panel of the Gateway (try http://telstra.gateway/ and see if that loads it) > Advance > Telephony > Information - Does that show anything about the serving being activated?
Most phones should be compatible with being connected to the voice port on the T-Gateway (or Gateway Max), it doesn't have to be a DECT phone, but DECT phones do allow you to connect it wirelessly a little easier.
Are you just not having any dial tone or any response or is there something specifically that isn't happening?
Are you able to access the control panel of the Gateway (try http://telstra.gateway/ and see if that loads it) > Advance > Telephony > Information - Does that show anything about the serving being activated?