To troubleshoot this further find out what conference bridge is being used for the meet-me conference when HQ starts the meet-me. The conference bridge most likely will be your HQ conference bridge, but you can easily confirm by starting a meet-me conference and browsing to the phone's web page (web access has to be enabled) and looking at the remote IP address under Stream 1. Once you know the conference bridge that is being used, telnet or SSH to the conference bridge and source ping the remote site phones making sure to source the pings from the SCCP local interface listed in your configuration. Most of the time no audio is due to IP routing or something blocking the RTP (UDP) traffic between the phones and conference bridge in this case.
