As far as I know, only messages. With the calls you can see the ringing.

I think so, but you might need a sim card.

There are various smartwatches that can accept phone calls when they are connected to the smartphone via Bluetooth. The Hama Fit Watch 5910 is not one of them, and I wouldn't recommend it either, because it is not sufficiently waterproof with IP68, at most it is splashproof.

This smartwatch Active S1 from Xiaomi runs much longer, is 50 m waterproof and can accept phone calls via smartphone using Bluetooth.

Xiaomi Watch S1 Active (46.50 mm, polyamide)