WhatsApp calls not working anymore

Dear AndroidPit people,

I have a very new issue with WhatsApp (and Skype) calls, that's been going on since yesterday.

I am living in Ireland and have been using WhatsApp on the Samsung Galaxy S5 for a long time, never had any issues, works perfectly fine.

Lately I have been doing a lot of phonecalls with a friend in Switzerland which worked perfectly fine. She is usually in proper wifi connection (with a HTC One or similar, definitely HTC) whereas I am on full 4G, this has worked very well for about 2 weeks.

Nevertheless, since yesterday (actually since the new Privacy Settings) the calling is not working anymore at all :
- if I try to call her the app sounds the ringing tone and shows that it's connecting. And it just stays on that indefinitely, it doesn't ever connect and there's nothing happening. My friend on the other side doesn't see anything happening.
- if she tries to call me I don't see anything happening and she receives the information that I am busy or already in a call and that I can't answer her call (something like that)

We have both tried to improve our connections but it doesn't make any difference. On rare occasions it happened that she called me and I could answer the call, however it immediately shows again that the app is trying to connect until the call drops. Neither of us can hear the other one...

The most curious thing now is : we have tried swapping to Skype as we couldn't figure out what's going on with WhatsApp. And low and behold, the exact same thing happens there!

So obviously the issue is somewhere either with my or her phone, but I couldn't tell what the issues is. As mentioned, it worked fine on multiple occasions during the last two weeks. The issue has only been happening since yesterday (Monday, 29th of August 2016).

Also, text messages work perfectly fine both on WhatsApp and Skype...

It would be great if someone had some idea how this issue could be resolved, as international calls between Ireland and Switzerland are pretty expensive... And I have a feeling the issue might also exist with different VoiP providers (Viber and so on...), as it's happening with Skype and WA!

Any help is greatly appreciated! Let me know if you need any further details or anything to figure this out...

Thank you very much,
David