I wouldn't build a phone patch, when so many are available very cheaply second hand. Heathkit and lots of others made thousands and thousands of them in the 1960s and 1970s and they don't need any sort of "controller," they are simply VOX operated, so once you engage the patch, it's "hands free" operation.
I see them on the market all the time for about $25. I couldn't homebrew one for that. They include hybrid isolation transformers (to impedance match and prevent ground loops) and level adjustments for the rig and the telephone. Just plug them in and use them.
That's all "third party" traffic, and you are the control operator once it's engaged, so your responsibility is to make sure you ID every ten minutes and monitor the conversation to assure it's of a personal nature and doesn't include profanity, music or other things that are prohibited.
A government which robs Peter to pay Paul can always depend on the support of Paul.
-- George Bernard Shaw