There are a couple of issues to consider (coming from one who tried to rig and then build a phone patch) :
I have learned that the "proper"
way to get phone conversations into a mixer and thus on the air is a dedicated phone patch and preferably a digital hybrid(which will give you the ability to separate and control the volume differences between caller and callee and also deals with the hum issues). Also, a hyrbid (depending on your mixer setup and use or lack of use of line-rec) can allow you to speak to a caller using your studio mic.
One (not the one I have) is here: