Re: ATA
Personally, I swapped out my ISP's broadband router for an AVM Fritzbox, which has analogue and ISDN phone ports, an answering machine and can act as a DECT base station, in addition to the normal router/switch functions.
In the past, I have used a small Linksys ATA and a Siemens Gigaset DECT ATA with an answering machine (but the voice menus were in German). Beware of any devices that are locked to specific VoIP providers.