What about a FritzBox?
I have been using an AVM FritzBox for the last few years. I have VOIP accounts with two providers, Voipcheap.com and Orbtalk, both of which are PAYG. The Orbtalk one at the time gave a free geographic UK tel no: other SIP services may give you the same.
In the UK I have a Fritzbox which, as well as being an ADSL WiFi+fixed router modem, also takes the POTS line (ie normal phone line) and the two VOIP services and allows multiple DECT handsets to phone out and take incoming calls on any line. It will also answer any line and listen for Faxes if wished, which it will then answer and forward to an email address, and/or take voicemail messages, which it will again forward to an email address. It can also forward incoming POTS calls to the VOIP system.
If I am abroad with an internet connection, I can then either use a computer with VOIP or else a Linksys VOIP system to connect to the Orbtalk and VOIPcheap systems.
PBW