Sometimes freeconferencecall.com has poor sound quality. This is particularly likely when the number of callers is large.
In this situation, their FAQ recommends states:
How do I eliminate static or echo on my conference calls?
If you experience static or echo on your conferencing line, please use the *5 key, to mute all callers. For participants that wish to speak have them press *6 to un-mute their line. Please refrain from using speakers, headsets, cordless phones, etc., if you encounter static or echo on your conference calls.