Newbie question... I have setup a 3CX server and configured the firewall with all necessary static NAT mappings. The server is reachable as "external.mydomain.com" with default ports. I have tested several softphone apps (3CX, Xlite) and these work just fine. Now I am trying to connect a Linksys SPA942 over the Internet to an extension. The Linksys is behind a NAT router, but I have enabled NAT mapping features. The phone DOES register on the server, but when I try to dial to this extension, I get the error "401 Unauthorized": (PS: Another Linksys phone hooked up to the LAN of the 3CX server and registered on the same extension DOES ring.) Extract from the log: ------------------------- 20:06:17.331 StunClient:nInitTests [CM506001]: STUN request to resolve SIP external IPort mapping is sent to STUN server 18.104.22.168:3478 over Transport <my-internal-server-ip>:5060 19:56:49.070 Call::Terminate [CM503008]: Call(9): Call is terminated 19:56:45.632 CallLeg:nFailure [CM503003]: Call(9): Call to sip:firstname.lastname@example.org has failed; Cause: 401 Unauthorized; from IP:22.214.171.124:40676 19:56:44.898 CallCtrl:nSelectRouteReq [CM503004]: Call(9): Calling: Shared:503@[Dev:sip:503@<my-internal-linksys-phone-ip>:5060, Dev:sip:email@example.com:40676] 19:56:44.898 CallCtrl:nSelectRouteReq [CM503010]: Making route(s) to [sip:503@<my-internal-server-ip>] 19:56:44.898 Extension:rintEndpointInfo [CM505001]: Ext.501: Device info: Device Identified: [Man: GrandStream;Mod: GXP-2000;Rev: General] Capabilities:[reinvite, no-replaces, unable-no-sdp, recvonly] UserAgent: [Grandstream GXP2000 126.96.36.199] Transport: [sip:<my-internal-server-ip>:5060] 19:56:44.867 CallCtrl:nIncomingCall [CM503001]: Call(9): Incoming call from Ext.501 to [sip:503@<my-internal-server-ip>] I don't understand why this works with softphones and not a hardphone... What am I missing?