smartphone - Windows (Nokia Lumia) 640 LTE
Operating System - Windows 8.1,
SIP account - Multifon (Megafon),
password have already checked - does not help
stun-server have switched on and off - does not help
Error 403 is returned from the server when it understands the request, but for some reason it cannot fulfil it, while error 408 is returned when the server cannot produce a response within the suitable amount of time. Since both errors come from the server, in your case it would be best to contact your VoIP provider for more information about these two.