Zoiper

If you are connected to WiFi everything works as expected, but if you are on 4G/3G you can not register because DNS errors

On 8.1 it will only receive while active, at lock screen, or standby. Will not receive while at menu or another program.

Zoiper is quite good. I am happy using it. I have had few rare experiences where it has connected me to the wrong person though while dialing the same number but most of the times it connects quickly & correctly. Good service.

When incoming call, nothing happened while in background Lumia 1520 wp8.1

Continue to develop it to get 5 starts. Bad thing it cannot receive calls because of push notification. A lot improved recently. Best Windows Phone SIP client.

Very usefull app. Need resolve the push notifications in windows 8.1. The app not working in background.

Account settings are difficult to set and reset themselves periodically (seems buggy). No way to manage certificate authorities or allow self-signed certificates.

Finally have SIP on my WP!