i have set up this to test your needs:
10004 Line to ext. 203
rule for 203 on unregistered forward to 200 (and 203 is unregistered)
then i called the phone number associated with line 10004 and look what happens and 200 was ringing.
i have included the logfile
>>>>>>>>>>>>>>>>>>>>>>>>>>
Time Function Message
06:48:16.019 Call::Terminate [CM503008]: Call(57): Call is terminated
06:48:15.988 LineCfg::getInboundTarget [CM503011]: Inbound out-of-office hours' rule for LN:10004 forwards to DN:203
06:48:03.879 Extension:

rintEndpointInfo [CM505001]: Ext.200: Device info: Device Not Identified: User Agent not matched; Capabilities:[reinvite, replaces, able-no-sdp, recvonly] UserAgent: [Grandstream GXP2020 1.1.4.17] Transport: [sip:99.88.25.24:5060]
06:48:03.879 CallCtrl:

nAnsweredCall [CM503002]: Call(57): Alerting sip:
[email protected]:63466;transport=udp;user=phone
06:48:03.769 LineCfg::getInboundTarget [CM503011]: Inbound out-of-office hours' rule for LN:10004 forwards to DN:203
06:48:03.754 MediaServerReporting::SetRemoteParty [MS210002] C:57.2:Offer provided. Connection(transcoding mode): 99.88.25.24:9014(9015)
06:48:03.488 CallCtrl:

nRerouteReq [CM503005]: Call(57): Forwarding: Ext:200@[Dev:sip:
[email protected]:63466;transport=udp;user=phone]
06:48:03.488 Call::RouteFailed [CM503014]: Call(57): Attempt to reach [sip:
[email protected]:5060] failed. Reason: Not Registered
06:48:03.488 CallCtrl:

nSelectRouteReq [CM503015]: Call(57): Target is not registered: Ext:Ext.203
06:48:03.488 CallCtrl:

nSelectRouteReq [CM503010]: Making route(s) to [sip:
[email protected]:5060]
06:48:03.488 MediaServerReporting::SetRemoteParty [MS210000] C:57.1:Offer received. RTP connection: 217.10.67.13:14606(14607)
06:48:03.488 CallLeg::setRemoteSdp Remote SDP is set for legC:57.1
06:48:03.488 Line:

rintEndpointInfo [CM505003]: Provider:[0991580098012 SIPGATEde] Device info: Device Not Identified: User Agent not matched; Capabilities:[reinvite, replaces, able-no-sdp, recvonly] UserAgent: [] Transport: [sip:99.88.25.24:5060]
06:48:03.488 LineCfg::getInboundTarget [CM503011]: Inbound out-of-office hours' rule for LN:10004 forwards to DN:203
06:48:03.426 CallCtrl:

nIncomingCall [CM503001]: Call(57): Incoming call from 09920792437@(Ln.10004@0991580098012 SIPGATEde) to [sip:
[email protected]:5060]
06:48:03.191 LineCfg::getInboundTarget [CM503011]: Inbound out-of-office hours' rule for LN:10004 forwards to DN:203
06:48:03.160 CallLeg:

nNewCall [CM500002]: Info on incoming INVITE:
INVITE sip:
[email protected]:5060;rinstance=12635708152e5859 SIP/2.0
Via: SIP/2.0/UDP 217.10.79.9:5060;branch=z9hG4bK3dd1.a6d5bb44.0
Via: SIP/2.0/UDP 172.20.40.3;branch=z9hG4bK3dd1.a6d5bb44.0
Via: SIP/2.0/UDP 217.10.79.9:5060;received=217.10.68.222;branch=z9hG4bK6b870284
Via: SIP/2.0/UDP 217.10.67.13:5060;branch=z9hG4bK6b870284;rport=5060
Max-Forwards: 67
Record-Route: [sip:217.10.79.9;lr=on;ftag=as48ca2af7]
Record-Route: [sip:172.20.40.3;lr=on]
Record-Route: [sip:217.10.79.9;lr=on;ftag=as48ca2af7]
Contact: [sip:
[email protected]]
To: [sip:
[email protected]]
From: "09920792437"[sip:
[email protected]];tag=as48ca2af7
Call-ID:
[email protected]
CSeq: 102 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Supported: replaces
Content-Length: 0
<<<<<<<<<<<<<<<<<<<<<<<<<<
this log is is very diferent to yours - it seems there is something not ok
it looks like 802 is one of your internal extensions.
what is about 800 is this DR or CallQueu or RingGroup?
Andy