Thank you all for your kind replies.
The problem still exists even after reset to factory defaults and setting the reg time to 300.
Here is the registration log:
06:58:31.984 FaxCfg::updateContact [CM504009]: Fax Service: changed contact to sip:
[email protected]:5100;user=phone
06:58:25.156 Extension:

rintEndpointInfo [CM505001]: Ext.100: Device info: Device Identified: [Man: Linksys;Mod: SPA-921;Rev: General] Capabilities:[reinvite, no-replaces, able-no-sdp, recvonly] UserAgent: [Linksys/SPA921-5.1.8] Transport: [sip:172.17.77.140:5060]
06:58:21.343 CallCtrl:

nIncomingCall [CM503001]: Call(1): Incoming call from Ext.100 to [sip:
[email protected]]
06:55:46.203 ExtnCfg::updateContact [CM504001]: Ext.802: new contact is registered. Contact(s): [sip:
[email protected]:40600;rinstance=a238366baf16a632/802]
06:55:45.921 ExtnCfg::updateContact [CM504001]: Ext.801: new contact is registered. Contact(s): [sip:
[email protected]:40600;rinstance=2f624367c2577f73/801]
06:55:45.296 ExtnCfg::updateContact [CM504001]: Ext.800: new contact is registered. Contact(s): [sip:
[email protected]:40600;rinstance=a51a122c40331555/800]
06:55:44.671 ExtnCfg::updateContact [CM504001]: Ext.999: new contact is registered. Contact(s): [sip:
[email protected]:40600;rinstance=f32b675cb022e84a/999]
06:53:48.562 FaxCfg::updateContact [CM504008]: Fax Service: registered as sip:
[email protected] with contact sip:
[email protected]:5060
06:53:44.968 ExtnCfg::updateContact [CM504001]: Ext.101: new contact is registered. Contact(s): [sip:
[email protected]:5060;user=phone;transport=udp/101]
06:53:43.828 ClientRegs:

nSuccess [CM504004]: Registration succeeded for: 10003@SIPNUMBER
06:53:43.640 ClientRegs:

nSuccess [CM504004]: Registration succeeded for: 10004@ORBTALK-UK
06:53:43.609 ClientRegs:

nSuccess [CM504004]: Registration succeeded for: 10002@NETMASTER VoIP
06:53:43.593 ClientRegs:

nSuccess [CM504004]: Registration succeeded for: 10001@IDILIS VoIP
06:53:43.484 ClientRegs:

nSuccess [CM504004]: Registration succeeded for: 10000@ZAPP VoIP
06:53:43.484 ClientRegs::Register [CM504003]: Sent registration request for 10004@ORBTALK-UK
06:53:43.468 ClientRegs::Register [CM504003]: Sent registration request for 10003@SIPNUMBER
06:53:43.453 ClientRegs::Register [CM504003]: Sent registration request for 10001@IDILIS VoIP
06:53:43.437 ClientRegs::Register [CM504003]: Sent registration request for 10002@NETMASTER VoIP
06:53:43.296 ClientRegs::Register [CM504003]: Sent registration request for 10000@ZAPP VoIP
06:53:42.921 ClientRegs:

nSuccess [CM504004]: Registration succeeded for: 10003@SIPNUMBER
06:53:42.875 StunClient:

rocess [CM506002]: Resolved SIP external IP

ort (89.136.157.14:5060) on Transport 172.17.77.140:5060
06:53:42.812 ClientRegs:

nSuccess [CM504004]: Registration succeeded for: 10004@ORBTALK-UK
06:53:42.640 StunClient:

nInitTests [CM506001]: STUN request to resolve SIP external IP

ort mapping is sent to STUN server 75.101.138.128:3478 over Transport 172.17.77.140:5060
06:53:41.562 ClientRegs:

nSuccess [CM504004]: Registration succeeded for: 10001@IDILIS VoIP
06:53:41.562 ClientRegs::Register [CM504003]: Sent registration request for 10003@SIPNUMBER
06:53:41.453 ClientRegs::Register [CM504003]: Sent registration request for 10004@ORBTALK-UK
06:53:41.406 ClientRegs::Register [CM504003]: Sent registration request for 10001@IDILIS VoIP
06:53:32.781 FaxCfg::updateContact [CM504008]: Fax Service: registered as sip:
[email protected]:5060 with contact sip:
[email protected]:5100;user=phone
06:53:32.171 ExtnCfg::updateContact [CM504001]: Ext.*1: new contact is registered. Contact(s): [sip:*
[email protected]:40000;rinstance=2338ba6f0d1a1b5f/*1]
06:53:31.578 ExtnCfg::updateContact [CM504001]: Ext.*0: new contact is registered. Contact(s): [sip:*
[email protected]:40000;rinstance=d42b41229c2dd44d/*0]
06:53:29.328 CallMgr:

nAddIPs IP(s) added:[172.17.77.140]
06:53:18.625 DBA [CM501003]: Connected: Database
06:53:17.171 CallMgr::findLocalIPs [CM501006]: Default Local IP address: [172.17.77.140]
06:53:16.625 CallCtrl::thread [CM501007]: *** Started Calls Controller thread ***
06:53:16.625 CallMgr::Initialize [CM501002]: Version: 6.0.650.0
06:53:16.625 CallMgr::Initialize [CM501001]: Start 3CX PhoneSystem Call Manager
06:53:16.406 MediaServerConnected [CM112000] Media Server is connected
06:53:16.218 LoadLicenceInfo [CM501010]: License Info: Load Failed
Any other ideas would be greatly appreciated.