I am using an Iphone 6 Plus. When I try to place a call it says the call can not be completed error code 312. On the screen it shows forbidden 403. I used to be able to make calls in the past, I haven't changed anything other than updating the IOS.
Error 403 means the server rejects the call.
The reason could be wrong username and password, wrong codecs, lack of funds, etc.
It is best to contact your provider to know what the reason is.
But what is error 312
Hello,
The error you receive is returned from the server, so it is best to contact your VoIP provider for more details on that matter.