Some years ago, I had the same "problem" and ended up with buying from UK a Panasonic KX-T616 and one programming telephone for this. Later I have seen many other options, but still the KX-T308 or KX-T616 seems to be the most reliable and easy to set up system accepting pulse and tone dialing. The UK version has UK (double) ringing when someone calls from an external line, and single rings on internal calls. The non UK versions does it the opposite way.
An exchange like this may be connected to the old telephone line, to an ip telephone adapter or a pc to phone adapter. In UK you have plenty of members of
C*NET and it is free. It is lots of other free or lo-cost networks. (Free if you have internet, and buy the adapter)
dsk