SIPNumber.com is offering free incoming US Numbers for subscribing to their service. This is currently the service I use and can recommend it since it is very reliable. The service is rarely down and sends any voice mail messages to my email.

How to get a SIPNUmber free incoming US Number?
Sign up at SipNumber.com. You will have to download their free softphone x-lite which is available free of charge on SipNumber’s website.