Linphone is a free app that let you use Messagenet VoIP on a Windows Phone smartphone.
You can download the app following this link: http://www.windowsphone.com/en-us/store/app/linphone/99661466-8c5c-489b-a567-569c1f480d29
- Settings > SIP account
Username > insert your URI (or internal number)
Auth userid > insert your URI (or internal number)
Password > insert password of your URI
Domain > sip.messagenet.it
Proxy > sip.messagenet.it:5061
Outbound proxy > Off
NOTE: currently (v184.108.40.206) the app don't work in background. Therefore you can receive calls only with app open in foreground.