A guide is available for download showing the key layout for each of the telephones.
Please see the M3900 series telephone guide, below:
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The M3901 telephone doesn't have a display. This means that features that rely on the display cannot be used.
Only the M3903, M3904 and M3905 have context sensitive keys. These will be set on the keys automatically when the TN is set up.
For full details see Communication Server 1000 Telephones and Consoles Fundamentals.