The Gigaset S850HX is a separate handset that you use next to your base station. You can be reached in two different ways: via the fixed analog network and via internet telephony, or VoIP. To make internet calls, you must set up 1 or more SIP accounts. The possibilities that you have with it depend on the provider. With multiple handsets you can simultaneously make 2 VoIP calls, which can be useful in a small business environment.
Please note: This handset can not be used separately. The device only works with a base station.