Question about Gigaset Dx800a

It is not known - at least the instructions say nothing about it. However, it depends on the configuration of the telephone network whether there are possibilities. With (old) analogue telephone connections or ISDN there is probably no possibility. With VoIP, you can use a router to which DECT telephones can be connected. Or you can connect an appropriate box (e.g. Fritz!Box) to the network - but then you might need another provider to make the telephony available. With a second IP connection (from the SIP provider) you can also serve a second number, then you have two separate networks, each with its own telephone number. Professional telephone systems can of course serve more "mobiles", but that would be a new price category.