Ticket Forms and Talk

We have 2 brands, and each brand has their own form. Both brands use Talk and each brand has their own phone number. One of the two brands is not yet live so I am trying to get the following question answered before it does:

When a call comes into a certain Talk phone number how does Talk/Support decide which ticket form it will use? 

I've asked this question of Support and rather than a straightforward answer I was sent a bunch of article links. I know that I have plenty to set up for the new brand before it goes live and I've been all through the articles. Unfortunately, I don't seem to see anywhere the describes precisely how I can ensure the correct form is opened when new tickets are created by Talk for the new brand.

I must just be missing something so any guidance would be extremely helpful.