Avaya Trimline - Manual (Page 4)

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Modular Outlet Requirement
You must have a standard modular telephone jack, such as one of those
shown below, to install your phone. If you have one of these jacks already
mounted, no tools are needed.
FIGURE 1 Standard Modular Telephone Jacks
If you do not have a standard modular telephone jack, contact your local
telephone company or Avaya Inc. for details on converting your present outlet
to a modular jack.