There are about a million of them.
Any VOIP box that have a device with a FXO and A FXS port will do.
Look for boxes that will work without a gatekeeper, in what is called peer to peer enviroment.
There are amny boxes out there, but you will do yourself a good service if you stay with a brand for both ends.
There are many claims that do not work out on the real world.
Q telnet will do it fine, quintum A400 will do it too. But you will have four ports, and I do not know if you need that many.