If you want to do VOIP with UPC with this S850HX, you should have some idea how the VOIP configuration works. UPC does not support this model, in fact none from Gigaset anymore. It works for me thanks to a bottle of wine and some good coaxing. A technician configured the UPC side in such a way that I was able to establish the connection on my side using the necessary configuration. Super device, but be careful. Without a clue, I strongly advise against it.

I think not. The box would have to support DECT, which I don't think is the case with UPC. If you have a box with DECT, it should be possible. But I have no practical experience with it.