Have a look at
www.sipgate.com
I use them since a few months, as well as their german affiliate sipgate.de.
Assigning severals devices to one "physical" number is no problem for sipgate.
Nice ui (
http://awesomescreenshot.com/0b9klgi4c ) + no running costs (apart from outgoing calls.
I am using sipgate.com to receive calls on my US number in germany, really great quality!
EDIT: Sorry, no number porting - at least I couldn't find something about that on their website. You could solve this by using google voice and then forwarding calls from GV to the sipgate number.