Generally, you only need one handset for a small house or apartment. In a bigger home, for convenience, you might want multiple handsets in different rooms – such as the hallway, living room, kitchen, or bedroom. However, with modems and NBN boxes, there is usually only one active port.
This means that you can only connect one corded phone, unless you have an adapter that can connect two phones at once to the single port. This also applies to cordless NBN phones, because the base unit still needs to be connected even though the handset itself can be removed.
However, some secondary cordless base units may themselves be wireless, meaning you can have another handset in a different location without having to plug it directly into the modem or NBN box. Having a single cordless base plugged into the port with multiple handsets connected wirelessly is the easiest way to go if you really want more than one landline phone in your home.
Even if you have more than one existing telephone socket, these will become inactive when you upgrade to an NBN connection, when you’ll only have one main socket. You can pay to have the wiring upgraded and replaced to allow more connections, but this can get expensive.