Cyrus, getting telephone audio on the air has always been a problem for broadcasters. To do it right you need a piece of equipment that separates the caller audio from the audio you are sending down the phone line. Usually this audio is your mic and maybe music/commerical or whatever audio you want your caller to be able to hear. This box can run from $500 to $3000.
Assuming you are trying to setup an interview here is a trick you use that I works just fine.
Take a speaker phone and put it near you so that your caller can hear your voice clearly. Then take a second microphone into your mixer and place the mic next to the speaker on the speaker phone.
No on your mixer you should have mic 1 which is your normal broadcast mic and mic 2 which is the caller/speakerphone audio. Play with the levels to get the balanced but keep your mic (1) the dominate signal.
Now the problem is if you want the person on the phone to hear another source other than your voice. The caller will only be able to hear what is going on via the speaker phone. So any audio coming out of your mixer will not be able to be heard on the phone.
You will be surprised how well this works.
I have used this trick many times and as long as only want to do interviews, this will be fine.