This is a pretty good question, I myself would like to know this as so far this is all I am aware of.
Octo Lines are not supported on the SPA-500 series
Octo Lines are supported on the 7900 series 9000 and 8000 series phones (Of which the latter two have not been ported yet if I am not mistaken)
Octo Lines are good if you want to run a shared DN and have up-too 8 incoming calls on that DN and then overflow it to another Shared DN set as Octo Line
Octo Lines are good if you want to have more than 2 calls come in on any one button, so effectivley you can have more than one button on the phone accepting up too 8 calls per button
Octo Lines are good for Receptionist environment, but only work with 7900 series phones and above, perfect for the buttons on the handset programmed for this and the side cart modules for monitoring extensions
Tha above is what I have or still is using them for, they can also come in handy for some other smaller features which I have never used and cannot see a need for it, whisper is one of them I believe.
It is a pity the SPA phones do not support Octo but there might be a practical reason for this
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