I'm using an AVM FritzBox 3790 as an ATA so I can use a cordless phone as na extension of the 3CX PBX. We have about 50 Cisco SPA504G's and a few other cordless phones connned using a combination of PAPT2's and a Billion 7404 VNPX. All connect and work without issue. In the last week I have been experimenting with FritzBoz as it has an ISDN (Basic Rate (BRI)) FXO along with two PSTN FXO's. While I can get them to connect to 3CX and work without issue. Every 10 minutes or so I get the following in the 3CX activity log 25-Feb-2013 23:04:43.854 [CM102001]: Authentication failed for AuthFail Recv Req REGISTER from 123.X.X.X:61274 tid=18896E5E3949FDE5 Call-ID=3C85D4D41E8024A9@123.X.X.X: REGISTER sip:3cxserver.x.com SIP/2.0 Via: SIP/2.0/UDP 192.168.178.28:61274;rport=61274;branch=z9hG4bK18896E5E3949FDE5;received=123.X.X.X Max-Forwards: 70 To: <sip:12712@3cxserver.X.com> From: <sip:email@example.com>;tag=2805978361 Call-ID: 3C85D4D41E8024A9@123.x.x.x CSeq: 866 REGISTER Accept: application/sdp, multipart/mixed Accept-Encoding: identity Allow: INVITE, ACK, OPTIONS, CANCEL, BYE, UPDATE, PRACK, INFO, SUBSCRIBE, NOTIFY, REFER, MESSAGE, PUBLISH Proxy-Authorization: Digest username="12712",realm="3CXPhoneSystem",nonce="414d535c073bde0749:3fe7f8bbbfd9a4f76e232eafa9829788",uri="sip:3cxserver.x.com",response="895f06486c2297736ac89c959ddf5313",algorithm=MD5 Supported: 100rel, replaces User-Agent: AVM FRITZ!Box Fon WLAN 7390 84.05.22 (Apr 20 2012) Allow-Events: telephone-event, refer, reg Content-Length: 0 ; Reason: License limit, processing or network error 25-Feb-2013 23:04:43.853 Exception: SipMessage::Exception Missing header Contact @ SipMessage.cxx:1285 Despite this message, the extensions remain registered and oprative. We don't have any license limt issues. Any ideas would be great.