if you have 1 co and 1 slt port on each system, put 1 fxo port in fron 1232 and have the fxs port on other end connect to line port
do the reverse on the second unit

I have used 2 Cisco SPA3102, one on each end, setup to pickup the extension on one side when picking up co port on the other.

if you have 2 of these units on each side you can have direct dial from extension to extension on each side

you can also set it for when you ring the slt extension, it will ring the co on the other side, easiest way to transfer calls if you don't have reverse polarity or cpc on your system