Hi all, This is my first post so please go easy on me I have a Siemens Gigaset A580IP phone which I would like to connect to our company's 3CX v10 PBX. I am remote to the PBX. The broadband connection the PBX is connected to does not have a static IP - we use DynDNS for host resolution. My broadband connection also has a dynamically assigned IP address. I have configured the phone with all authentication credentials and the dyndns address of the server - it attempts to connect but the server rejects the connection with the following error: 14:50:09.402 [CM102001]: Authentication failed for SipReq: REGISTER 192.168.1.250 tid=f7601dd8e88074bcd18515ac363055e1 cseq=REGISTER email@example.com:5060 / 44 from(wire); Reason: Credentials don't match, check that authorization-ID and password match the ones in extension settings I have heard mention that I may need to use STUN but I don't have a clue where to start with this - can somebody point me in the right direction please? 3CX softphone is working perfectly in tunnel mode so I assume all ports have been forwarded corectly - firewall checker is passing all tests also. Many thanks in advance, Paul.