When someone dials an extension or a second call comes in from a DID assigned to that extension the call goes directly to voicemail instead of ringing the next line appearance on the phone. All phones have 3 line appearance buttons all setup for line1 on the phone. The phone is a Yealink T57W. This does not appear to happen all the time however I have re-produced it.
FreePBX 16.0.40.11
You need call waiting enabled for the extension, toggle with the call waiting feature codes.