I'd expect all DECT to work, though it kinda makes more sense IMO to go for a DECT base station that does VoIP itself - a waste of electricity to have two units.
There's also the fact VoIP is not "a phone line", as in if you have four handsets all four can make a call at the same time. With an ATA and DECT station plugged into that, you are limiting that to one. Probably not something I will ever need, but its one less thing to think about, plus I can always connect to VoIP over my mobile too.
I don't use an answering service, but if I did I can enable that at AAISP so they simply e-mail me the messages, no need to have anything at home for that. In fact, I have it set to record all calls as we both have a habit of forgetting what people said over the phone.