VoIP with various apps not possible after 6.0 upgrade

I have had my S6 for over half a year with no issues. Did the offered SW upgrade to 6.0 about 3 weeks ago and since then cannot do calls in WhatsApp, Skype or Messenger. I also have facebook giving me a 'not connected' message in some places.
Did a factory reset to no avail, it seems the firmware is faulty and it can't be reset. The Samsung store in London Oxford street claims not to be aware if this, but as luck would have a guy overheard me and his and his wife's S6 have the exact...

VoIP with various apps not possible after 6.0 upgrade

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