You can certainly do that. Using an FXO gateway such as
http://www.grandstream.com/products/gateways-and-atas/voip-gateways/product/gxw4104/4108
you wouldn’t even need a server at the remote site (the phones could connect to the headquarters PBX over the VPN).
However, in most applications that would not be a good design. POTS lines are generally much more expensive and less flexible than SIP trunks, but are often chosen because they are more reliable and are less likely to have voice quality issues. Having the remote site connect over the VPN negates those advantages – the POTS lines would be inaccessible in an internet outage and could have quality issues when the VPN is overloaded.
We could give better advice if you provide more details. What country are you in? How many extensions at each site? Number of POTS lines at each site? Are the POTS lines actual copper fed from a central office, or are they delivered over the same fiber or cable with your internet connection?
Do you also have SIP trunks? What kind of phone system does the remote site have now?