nope, thats abnormal.
do you have previous phoneline? normally double story houses would have a point each up and downstairs.

If you do, you could reuse it.

Else, get a splitter and run a cable from upstairs to downstairs

erm... landed? you dont have existing fixed line before apply UniFi?

I remain my existing fixed line when i apply UniFi, now i have two phone number, one is existing from the pole, another one is UniFi VOIP. But using existing phone line will be charged

my UniFi BTU is upstairs as well, but my landed house last time using 4 cores cable for phone line, i manage to make it from BTU phone socket <---phone cable---> my existing phone socket upstairs, connect the 2 spare cores <---> go down stairs open up socket and look for the 2 spare cores.... and connect with the phone, now my existing fixed line phone and UniFi VOIP is side by side down stairs 

only doable if your upstairs have phone socket, because house use to link up all phone socket so downstairs ring, upstairs also ring and also internal cable for the phone socket must be 4 cores... 2 cores you may choose to sacrifice existing fixed number if you have.

Buy same model cordless phone, can use as charger. You can even connect both to same base so both can use up and down.. Can get at garage sales.

why your photo the rectangle in red looks like ADSL filter/splitter??

your UniFi for landed not direct fiber to your house? and Unifi phone is VOIP, connected directly to your UniFi BTU, nothing to do with the existing phone

If you want to buy new 1, than buy the model with 2 handset no need to configure anything.

What I mean is, you buy another cordless call it B. Existing is A. A is connected to unifi BTU. B is not connected to anything. You just want to use as a charger. Instead you put A handset into B charger (base) you can configure B to connect to A base. So both can make/receive a call.