Line 1 is the default/prime line. If you have 3 cell phones connected, and go offhook on an extension set, you will be using the cell phone that is paired with Line 1 (indicated by the single dot on the top of the XLINK).

Each blue button on the top of the XLINK is a cell phone line number. The dots indicate which is line 1, line 2 or line 3.

If you do not pair a cell phone to all 3 lines, XLINK will always use the lowest line number for outgoing calls. For example, if you only have a cell phone paired to line 3, then go off hook on an extension set, line 3 will be the prime line. If you have a cell phone paired to line 2 and line 3 and go off hook on an extension set, line 2 will be the default outgoing line.

It is a good idea to pair the cell phone you will use the most to line 1