When we run the firewall check (that does not lie...), we always get the following result:
resolving 'stun2.3cx.com'... done
resolving 'stun3.3cx.com'... done
resolving 'sip-alg-detector.3cx.com'... done
testing 3CX PhoneSystem 01 SIP Server... failed(How to resolve?)
stopping service... done...