The tier is irrelevant. It's totally random.
I've seen lines on fibre 900 with and without digital Voice, and lines on 80/20 with and without digital Voice.
I've just gone on 160/30 and they're activating my copper line later this week for calls.
I ordered the same package for my neighbour and they got put on digital Voice.
No amount of asking can have them change whether or not they provide digital Voice.
The support staff can't change it during the order process or after service is live.
Digital Voice requires the BT Smart Hub 2, which they sent me 2 of
but still no digital voice option.
Everyone on copper will eventually be moved to digital voice anyway.
It's also not an FTTP only thing now.
You can take out FTTC with BT and they can put you on digital voice.
Again there is no choice on this if that's how they want to provision you.