I have a Surface 3 LTE (not Pro) and I've connected it with T-Mobile DIGITS and the software adds a dialer. Yes, you can use it as a phone (and SMS) in this manner but you'd still need a phone line to mirror from. The GSM radios on the Surface are weak and this method lets me "tether" to my phone's superior radios.
Here in New Caledonia, we don't have several options. Only one SIM for calls + 4G . I would love to have a real computer in tablet form in my bag and a Bluetooth headset in my pocket to answer phone calls...
I don't know why no other tablet or computer took advantage of the Bluetooth MAP profile to send and receive phone calls and texts like you could with the HP Touchpad. I don't carry that tablet much anymore but it's one of my favorite features since my phone can stay tucked away while using the tablet. While webOS had many firsts, this was an "only". The BlackBerry Playbook could do SMS while bridged but no phone calls.