jovial is correct. your first connection out to homeplug1 should be from dlink and not the btu. while regarding the second phone setup, by my "logic" understanding/guessing (no any knowledge prove) I don't think dlink will carry the voip signal, because signal already been split by at the point of btu, and phone coming out from there only, so is a no..

it still can be done by the setup you propose, but is a different thing already, a standalone internet phone (not the same number from unifi).

use the line2 socket from the btu (can support fax), or get a twin cordless phone like jovial suggested.

[maybe] the phone signal can be redirect out through another lan port of the btu?