Do you lose your phone line when broadband contract runs out?
@nmdrex If your father is happy with the smartphone you probably only need the landline for backup/continuity.
As such a simple ATA is probably the best option until you/he decide it's actually superfluous.
There are pretty cheap/simple units such as the Grandstream HT801 or HT802 and slightly more functional such as GigaSet N300IP. The latter will also connect to the existing analogue phone line before transfer so is useful during transition.
What you choose depends on what handset(s) are needed to function, e.g. simple corded or DECT cordless and how many?
Setting up the kit and a local additional/temporary number (cheap enough with A&A) beforehand as recommended above is a good idea.
Personally I'd avoid the Fritzbox as it would seem that it's (admittedly excellent) capabilities would be OTT in this situation. Just stick with the PlusNet router as a separate ATA will work with any router. Some can be USB powered and a Hub 2 has an otherwise useless USB port (just saying...) 
