Note: this service is provided by CallCentric (www.callcentric.com)
You can order free US phone number from CallCentric and use it with your Sipmobile phone for incoming calls.
Go to this page: Free Phone Number and order a number (registration required).
Then in your CallCentric account setting set DID Forwarding to your Sipmobile number.
Use the following format firstname.lastname@example.org, where 991xxxxxxx is your Sipmobile number.
That is all. See an example: