In the current preview of the Skype for Business client, calls in from the PSTN are given a different set of controls in the call window (dial pad on by default, no Video or Present buttons, adds a Transfer button), unless the caller is a contact. If Skype can reverse look up the phone number in my contacts then it shows the same controls as it would for a Skype-to-Skype call.
This doesn't make sense. If it's one of my colleagues calling into the office from his cell phone, (via PSTN, not using the Lync 2013 Mobile client) the Video and Present buttons are useless. It's still a PSTN call, I can't add these things to it. Half the time staff call in from their mobiles they're calling the main number and asking to be transferred to someone, so having the dial pad and the transfer button show up makes all the sense in the world.