SIP-T21P_E2 refusing calls issue

[Alvaro]

Hello support

I am having an issue in a client which uses several T21_E2 phones
We are talking about a callcenter in which our system only send calls to agents when the phone is available,
We are facing the following issue:

Time to time an agent receives a call and the phone apparently refuses the call sever times form less than a second of time and finally the call gets connected
I attach an extract of the phone log and the complete log as well:

Sep 6 19:55:26 sua [492]: APP <6+info > [SIP] <IPC_ntfp>:msg=0x00040101(262401), wparam=0, lparam=32880, size=264
Sep 6 19:55:26 sua [492]: CAL <5+notice> [000] call status update [0]==>[1]
Sep 6 19:55:26 Log [539]: TALK<6+info > [MSG:SIP==>Talklogic] message=[SIP_CALL_INCOMING][40101] wParam=[0] lParam=[32880]
Sep 6 19:55:26 Log [539]: DEVL<6+info > netIsValidIP 192.168.100.11
Sep 6 19:55:26 Log [539]: CUIT<6+info > Refuse call
Sep 6 19:55:26 sua [492]: APP <6+info > [SIP] <IPC_rcv >:msg:0x00041103(266499), wparam:0x00000000, lparam:0x00008070
Sep 6 19:55:26 sua [492]: APP <5+notice> [SIP] SIP_C2S_CALL_RESPONSEE, lid:0 cid:32880, response_code:1
Sep 6 19:55:26 sua [492]: SUA <6+info > [000] ****API Event:(0x0002) SIPUA_EVENT_A2S_RESPONSE ****
Sep 6 19:55:26 Log [539]: TALK<4+warnin> Callwaiting is Off just refuse incoming call
Sep 6 19:55:26 Log [539]: CUIT<6+info > post SUA_MSG_CALL_RESPONSE
Sep 6 19:55:26 sua [492]: APP <6+info > [SIP] <IPC_rcv >:msg:0x00041103(266499), wparam:0x00000000, lparam:0x00008070
Sep 6 19:55:26 sua [492]: APP <5+notice> [SIP] SIP_C2S_CALL_RESPONSEE, lid:0 cid:32880, response_code:486
Sep 6 19:55:26 sua [492]: SUA <6+info > [000] ****API Event:(0x0002) SIPUA_EVENT_A2S_RESPONSE ****
Sep 6 19:55:26 sua [492]: CAL <5+notice> [000] call status update [1]==>[16]
Sep 6 19:55:26 sua [492]: DLG <5+notice> [000] Message sent: (to dest=192.168.100.11:5060 len=520)
Sep 6 19:55:26 sua [492]: DLG <6+info > [000]
Sep 6 19:55:26 sua [492]: DLG <6+info > [000] SIP/2.0 486 Busy Here^M
Sep 6 19:55:26 sua [492]: DLG <6+info > [000] Via: SIP/2.0/UDP 192.168.100.11:5060;branch=z9hG4bK08ac3826;rport=5060^M
Sep 6 19:55:26 sua [492]: DLG <6+info > [000] From: "FLIA BASCO" <sip:26138684@192.168.100.11>;tag=as099b0555^M
Sep 6 19:55:26 sua [492]: DLG <6+info > [000] To: <sip:300@192.168.100.122:5060>;tag=57859887^M
Sep 6 19:55:26 sua [492]: DLG <6+info > [000] Call-ID: 254028eb506d91602e2bfb456661ac70@192.168.100.11^M
Sep 6 19:55:26 sua [492]: DLG <6+info > [000] CSeq: 102 INVITE^M
Sep 6 19:55:26 sua [492]: DLG <6+info > [000] Allow: INVITE, INFO, PRACK, ACK, BYE, CANCEL, OPTIONS, NOTIFY, REGISTER, SUBSCRIBE, REFER, PUBLISH, UPDATE, MESSAGE^M
Sep 6 19:55:26 Log [539]: IDLE<6+info > Miss Call Count Change, [5] Miss Call(s).
Sep 6 19:55:26 Log [539]: CUIT<6+info > CStatusManager: add 12, 5
Sep 6 19:55:26 Log [539]: IDLE<6+info > CStatusManager: Notify Status Changed.

These cases are a bit randomly and happens to several phones
Until today we are not being capable of detect why the phone is refusing this calls, the agents are actually available when the issue happens


Someone has face this same scenario ?
Thanks in advance