Thread Speakers Feed sound back through Lapel, any suggestions?

I'm am running the audio for a church that is meeting in cinema theater.

I have not had too much experience with audio equipment and i'm pretty much learning as I go.

This church incorporates phone interviews into the service to get different views or a better understanding on different issues.

The setup: we run the cell phone through the mixer (so the person on the phone can talk through the cinema speakers) and then use an output to send conversation back through the phone (pastor's lapel to person on the phone).

Right now with this setup when the person on the cell phone speaks all lapels have to be muted so that their voice is not feedback through the pastors lapel. Since the lapels have to be muted the pastor or other speakers are not able to stop the person on the cell phone and ask for futher explanation on a paticular topic or just add comments.

Since the theater has surround sound there is not a neutral place where the lapel will be sufficient and not pick up the voice from the speakers. I know a handheld mic would work better, but we really perfer the lapels over the handheld. I was wondering if we could use some kind of filter or a different setup? I do have a limited budget, any suggestions will help

Thank You
cc