First of all I am not able to find check box of "billing same as shipping" on Register page of E-commerce . You can only add shipping address on register page. May be that check box is removed to keep things simple. So reported bug is invalid now.
Now I can see this case on check out page, i.e when clicking on "billing address same as shipping address", billing phone number is not populated . But this was never expected from this check box. Clicking on check will only copy address and not phone number. Also service on server side add "billing" as a purpose to the shipping address which gets created when this check box is checked. Service also does not care about phone number. So, this not a bug. You should fill the phone number at the time of creating billing address.
To answer the question asked by Kiran Gawde , OFBiz data model gives flexibility to manage every contact separately. Decision of UI is completely dependent on requirement and how your will like to define user experience. Different people may have different views on defining user experience.
Also its not necessary all the time that if billing and shipping address is same, then phone numbers will also be the same.
We should mark this issue invalid and close it.