Audio Feedback when using cell phone as alternative mic

[Scribner]

My computer's mic has poor pickup.  Application is a large room with a dozen participants and several remote.  I was advised by another user that I could substitute the mic on my cell phone for the one on the computer by dialing into the meeting, shuting down audio output on the cell phone, and turning off the mic on the computer. 

That is, the cell phone is in the same large room as the dozen participants and the computer.   I get too much feedback, I believe the loop is between the computer's speaker and the mic on the cell phone.

Can anyone think of a solution ?

Has anyone tried this successfully ?