I thought that I would mention that our modem/router/WiFi is an AVM FRITZ!Box 7590, now superseded by a later model, the FRITZ!Box 7590 AX.
It is also, amongst other things, a DECT base station and we have a number of portable handsets and mobile phones registered.
We also have a Fritz WiFi extender which seamlessly gives us WiFi in parts of the house where the signal was previously fairly weak.
As part of the functions there is not only Voicemail but also a register of incoming and outgoing calls and a blacklist system. It is possible to transfer calls from one handheld to another, or, for that matter, to the wired handset.
Highly recommended. Their UK site is
https://en.fritz.com/products/fritzbox/