We have a new v15 SP2 install going in. Adding the phones each user extension is taking up the first two buttons on each phone. We are using Polycom VVX410 and Polycom VVX 311 phones.
I seem to recall that being the case (the default) for a while now. The thinking is probably that , as a default, on a multi line set, the user will want to be able to take, or make, a second call, after putting the first on hold.