I've ended up putting together a little bit more complex setup that is really needed, with an Asterisk box at home connecting up to three different VoIP providers.
However, many years ago I bough some Bang&Olufsen cordless phones that I wanted to keep using. To hook these up with the home Asterisk set-up I am using a small Cisco box
 into which you connect your old analogue phone and a network cable. Over a web interface you can set all of your VoIP providers details. Something like this little box could be what you are looking for?
I also have CSipSimple
 installed on my Android phone, so that when a call comes in on the "land line" and I'm on a WiFi connection, my mobile will ring as well as the house phone.
For my Swiss number, the VoIP provider I'm using is netvoip
. They allowed me to port my old Swisscom number, and it's now a prepaid service. Free incoming calls, and ok pricing for outgoing.