This is an interesting question. If you would consider only the phone media, than the employees with a less number of the skills should get the inbound calls first just for the efficiency reasons. When it comes to the mix of different medias things are getting far more complicated. I guess there is no single best way to combine medias but there are several strategies and the choice depends on the mission and vision of the contact centre. Somehow I have an idea that employees would prefer to work a full day only on one of the medias but have slight media rotation during the week. From the speed of answer reason you could prefer to set employees on email or internet if the phone media on that day is overstaffed.
I would say, while making these choices and making the roster think broader and consider different interest in the organization, facilities to schedule and monitor the activities and your mission and the vision.