best response confirmed by ThereseSolimeno (Microsoft)
Hi, yes you can assign a Direct Routing number to a Common Area Phone using the normal Set-CsUser command with the -OnPremLineURI parameter. For a CAP you need to create a normal Office 365 user account in order to apply the CAP license.

In reference to the call forwarding, I cannot see this being an issue if you use a Teams native IP phone. In the phones firmware under the "Calling" section you can set forwarding actions if unanswered to another contact or number. I hope this helps.