extending PBX to remote office over IP

I would like to setup site-to-site vpn between our HQ and a remote office. Besides data I also would like to give a home office employee to be able to use the old-fashioned PBX phone so he can use it as if he was in the office.
How can I accomplish that? I have seen designs where two PBX's were connected together over IP network, but in my case the other side would be a regular pbx phone not a pbx.

Hi and welcome to Voipuser

Since we can obviously assume there is a DSL connection in each location it becomes just a question of adding an ATA or terminal adaptor, something along the lines of a PAP2 (but there are other alternatives).

A standard PSTN style phone plugs into it and it functions thereafter as an extension of the office system. Important to have a good quality connection with sufficient bandwidth for your computer data and voip data - it's possible to use a router with QOS which will ensure VOIP traffic gets priority so take a look at this option when specifying equipment.

I am pretty sure this has been covered a few times in the forum so some careful searching will bring up some further info.