Phone becomes louder with voice echo when caller in queue:

Hi everyone,

Have FreePBX with 2 HT-503 running off PSTN lines. When there is a caller with my receptionist, she notices a beep and subsequent voice echo when a second person joins the queue. I have also noticed it when a softphone joins the queue. Any advice?

Update: Upgraded FreePBX and resolved. Documented my set up here: