Surface Phone – ARM – Skype – limitation

Suppose a person owned a cell-phone-sized surface tablet (for portability) that ran on an ARM processor (for good battery life) that included 3G-LTE type of connectivity. Then what are the limitations of such a device’s ability to act as a mobile-phone using only Skype’s VOIP and a data plan. I seem to remember that Skype can receive calls to a pre-assigned phone number. I would wonder if such a setup would be a viable alternative to the typical smartphone? Lastly if this did work then would we even need a cell phone company like Verizon or ATT?