Conferences (meetme) sounds ugly if someone join from PSTN

Hi all,
Recently we added conferences module in our FreePBX v2.7.0.6, test with internal SIP phones sounds very good but if someone calls to the conference room from PSTN the conference starts to sound ugly. We created the conference room and throug inbound route we get this conference room access from outside world.

Thanks in advance.

Run FXO tune