Register an SL910
Here's a handy guide if you have just got a set of phones, or just an additional handset to supplement your current system. You will likely need to register your new phone with the main base unit. This is the same to register an SL910.
This pairing lets the phone tie in with your current phone system and allows you to extend the number of handsets you can use at home or in the office at any given time (up to the maximum the base can handle)
Here is a quick video on how to register an SL910 Handset from Gigaset
These steps will be very similar if pairing to an alternative base unit that is not an SL910 (although must be GAP compatible)
For other makes and models please see the here, for the most common phone types