I am trying to setup a Siemens S685IP phone with our hosted 3CX machine. The phone is located behind a m0n0wall firewall, with a 1:1 NAT setup. I am able to make calls from the phone, and I get full two way audio. However the phone will not receive a call. Here is a partial log from when the call is made: 07:52:03.908 [CM503008]: Call(21): Call is terminated 07:51:39.068 [CM503025]: Call(21): Calling Ext:Ext.401@[Dev:sip:firstname.lastname@example.org.*:3949] 07:51:39.068 [MS210002] C:21.2:Offer provided. Connection(transcoding mode): 64.34.*:9030(9031) 07:51:39.026 [CM503004]: Call(21): Route 1: Ext:Ext.401@[Dev:sip:email@example.com.*:3949] 07:51:39.025 [MS210000] C:21.1:Offer received. RTP connection: 81.187.*:15082(15083) 07:51:39.025 [CM503010]: Making route(s) to <sip:firstname.lastname@example.org.*:5060> 07:51:39.024 Remote SDP is set for legC:21.1 07:51:39.024 [CM505003]: Provider:[*] Device info: Device Not Identified: User Agent not matched; Capabilities:[reinvite, replaces, able-no-sdp, recvonly] UserAgent: [A&A] PBX contact: [sip:*@64.34.*:5060] 07:51:39.023 [CM503001]: Call(21): Incoming call from 07*@(Ln.10004@013*) to <sip:email@example.com.*:5060> 07:51:39.018 [CM503012]: Inbound out-of-office hours rule (unnamed) for 10004 forwards to DN:401 07:51:39.017 Looking for inbound target: called=013*; caller=07* 07:51:39.015 [CM500002]: Info on incoming INVITE: INVITE sip:013*:*@dolphin.fer*.com SIP/2.0 Via: SIP/2.0/UDP 81.187.*:5060;branch=z9hG4bK5c288e3a;rport=5060 Max-Forwards: 70 Contact: <sip:firstname.lastname@example.org.*> To: <sip:013*:*@dolphin.fer*.com> From: "07*"<sip:email@example.com.*>;tag=as1e7eaa0a Call-ID: firstname.lastname@example.org.* CSeq: 102 INVITE Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY Date: Fri, 01 Jan 2010 12:51:38 GMT Supported: replaces User-Agent: A&A Content-Length: 0 I have replaced sensitive information with an *, the log shows that all of these IP addresses are correct. So what is stopping the Siemens from answering it's invite request ? My firewall is setup log all blocked packets, and I can't see that it's blocking anything. Has anyone else got a Siemens Gigaset phone working from behind NAT ? Thanks for your help.