T42S Inbound Call Issue

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Hi Guys

I am working on a rate issue at the client today.

Issue:
One of the phones failed to receive inbound calls randomly. To look into it further, I've done a Pcap on the SIP server end to see how that phone behave. The result indicated the SIP server is sending call invites to all 5 phones. And, only the problematic phone responses to the invite with "SIP 302 moved temporarily" whereas the other 4 phones responses with "SIP 180 Ringing".

Here are the spec and setup:
> 5 x T42S (Firmware: 29.88.0.90)
> DND: disabled
> Busy call forward: disabled
> Always call forward: disabled
> Local Directory: 14 pages with approx 145 entries
> All phones are connected to the same VoIP Service, internet connection and local switch

Attempted Solution:
-> Firmware update (Failed to address issue)
-> Factory reset (Failed to address issue)
-> Change the local network port and cable for the connection (Failed to address issue)
-> Checked local directory - blacklist (It is already empty)
-> Deleted all entries in the local directory (It addressed the issue)

Any idea what is the possible cause and fix for this issue, if the customer still want to use the Local Directory?

Thanks!

Roy