Couple of options.
The simplest is to get a Gigasey Go-Box 100 which can handle both a
network connection (RJ45 for VoIP) and a standard RJ11 for PSTN. The do
need to be set up but AFAIK do the job. The GoBox 100 is also a DECT
base station in its own right.
In terms of phones that can handle PSTN and VoIP, look see what Yealink,
Avaya, Polycom and Snom have to offer. Look at onedirect.co.uk
seem to have a fair range. Also look at Cisco and 3CX who do ATAs and