Wolfgang, WP8 has really a lot of VoIP options. Not knowing your VoIP service, this is how I use it on WP8.
First of all, Skype is really well integrated on WP8. Call quality is incredible, free voice and video calls to any Skype user on any device (PC, tablet, mobile phone) with Skype installed. In addition although with some extra cost, you being an ex-pat like me you might want to buy a so called Skype number on each country you have friends and / or family who regularly call you to make their calls cheaper or totally free. Skype numbers are regular landline numbers. I have a Skype number in three countries, for instance my family back in my native Finland pay nothing for calls to me using my Finnish Skype number (landline calls in Finland are always included in your monthly basic fee):
When home and my niece calls on my Finnish Skype number all my devices currently online alarm me; I can then select if I take the call on Windows phone, Angie's iPad or my laptop or Windows tablet.
I also use a WP8 app
MobileVoIP for another VoIP service I am subscribing.
MobileVoIP can easily be found on
WP Store:
Install
MobileVoIP, create a new user using credentials of your existing VoIP service or sign up for a new service:
MobileVoIP website:
https://www.mobilevoip.com/
Viber which Brink told about above is totally free and works well. Install it and it will automatically recognize which of your phone's contacts has
Viber installed, thereafter calls, texts and picture messages between two
Viber users are free. Very practical, very easy to use and learn.
Your VoIP needs must be really special if you cannot find something which works for you on your new Windows Phone.
Kari