I currently have 3 Linksys SPA962 Phones, they are running the most recent version of firmware available (6.1.5a) and each have an individual line programmed to Ext 1 and are registered and working properly.
On Ext 2 for each of the phones I have a SLA built. In each of the phones I have the same number built with both the Phone (global) and Ext 2 (line) configurations set as "Shared" not "Private". Ext 2 on each displays ringing properly when an incoming call occurs and shows the line in use when it is answered by one of the three. The issue I have is when the call is answered and then placed on hold by any of the lines, it cannot then be picked up by either of the other phones.
Looking at traces for this when the call is attempted to be picked-up, SIP INVITE message shows just the IP address of the phone making the request, not the full <phone#@IPaddress> information.
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