Linphone is a free app that let you use Messagenet VoIP on a Windows Phone smartphone.

You can download the app following this link:

App configuration:

- 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 >
Proxy >
Outbound proxy > Off

NOTE: currently (v1.0.2.0) the app don't work in background. Therefore you can receive calls only with app open in foreground.