No it is not the phone handset that gives you the line capacity but instead it is the capacity of the Gigaset Base Station.
The Gigaset N300 IP Base Station allows for 3 consecutive lines. If a call is transferred this takes up 2 lines leaving just enough capacity to receive in 1 more line.
The Gigaset N510 IP Base Station allows for 4 consecutive lines to be in use but if you transfer a call this takes up 2 lines giving you 2 lines free.
If you need to look at increasing your call capacity then the best way would be to add in another Gigaset Base Station so that you have 2 base stations connected to your router(s) giving you more capacity to make and receive calls.