Lets look at the use-cases

Legend:

1. For the sake of illustration lets identify my Cellphone as 905Cell

2. Lets identify my home phone line as 905Home (in my case it is a Vonage line with a World Plan)

3. Lets identify my SIP based soft phone line as 905Soft 

Use-case 0:

905Home is my home phone and I use it to dial all over the world


Use-case 1:

1. 905Cell calls 905Soft
2. 905Soft gives me a dial-tone 
3. I dial any number I want as I would from my home phone line
4. Behind the scenes 905Soft switches my call to 905Home (described in "How" later. Hint: use Linksys SPA3102 device

Use-Case 1 for family :

My entire family all over N America can run use-case 1 from their own phones 

You want to get really fancy?
Search for and download a dialer app on your Android or Iphone device which automates calling 905VS and dials the actual destination number. Just like GV, right?
I recommend Dialer-X for Android.