I was going to recommend a Gigaset but you beat me to it. I think you'll be very happy with it (I presume it's coming with a handset). They can have up to six SIP registrations (VoIP numbers) as well as one normal phone line. I have one in my apartment in London and another in my place in the 'Med so I can make and receive calls on the same number at both locations and have free calls between them.
If you want to carry on using your existing handsets then the Cisco SPA series is worth a look but when it comes to configuration they're about as user-friendly as a cornered rat.