General Newb Questions - Any help is appreciated

I really have a simple vision here.

I have a windows 8 tablet, that I want to be able to take with me anywhere. This windows 8 tablet will have a custom application designed by me that will just simply do VOIP calls.

Being the IT freak that I am, I want to be able to handle everything for A-Z.

So basically this is my understanding. Since I want to be able to make calls to a LAN line, I need the following:

PBX server
DOD service
soft phone (which is the windows 8 tablet/ with app)

Am I missing anything? Is it also possible to create your own DOD service?

And if I wanted to receive calls I need a DID service correct? Can you create your own DID service?