Thanks so much for your reply.
I will be honest, much of what you said required a google but I think I have understood.
Have you tried switching services?
>>> I have switched to Skype through my PC and I am able to make calls without trouble. I have also used a different VOIP system called Keyyo through an old Macbook using the same internet connection without trouble.
Can you use an echo-test service?
>>> I can use one and have done so without a problem.
>the intermittent nature of your problem screams to me "NAT firewall
>>> I went into my router and found "NAT virtual server set up". There was a server set up under the name Whatsapp... I removed this and disabled NAT and I will run a few tests making calls tomorrow to see if there is any success.
In addition I checked the settings on Linphone and within the "settings" menu under "network settings" there are four options at the bottom of the menu page;
'direct connection to the internet'
'Behind NAT/Firewall (specify gateway IP)'
Behind NAT/Firewall(use STUN to resolve)'
Behind NAT/Firewall(use ICE)'
'direct connection to the internet' is selected and I have not changed this. Should I change it?
You just have to swear off IPv4 just like you have to delete your Facebook
>>>In the same "network settings" section mentioned above there is an option to tick "Allow IPv6". In the spirit of a process of elimination I will not select this option until I have tested the effect of changing the router, but it will be my next port of call.
Thanks again in advance for you help, as well as the sage wisdom about deleting Facebook.
Kind regards Grant Griesel.