Garbled Ring instead of MoH during Queue


I have a system running FreePBX Distro - with the following set up:

50 Extensions
12 Ring Groups
1 receptionist

The receptionist is the only member of the only Queue, and she is set as a Static Agent. The Queue is configured to ring instead of MoH. When calls come in they hit the receptionist. She in turn transfers the calls through to either an Extension or a Ring Group, then hangs up. If more calls are waiting in the Queue she answers and deals with them in the same manner.

The issue being experienced is if there are 2 or more people waiting in the Queue, the ring tone they hear is garbled.

Could this be to heavy Queue usage (although it seems unlikely), or could there be an issue with the ring tone sound file (again unlikely)? Any suggestions?

I’ve tried to tweak the Queue by setting Agent Restrictions to Extensions Only and Skip Busy Agents to Yes + (ringinuse = no) - just to see if that helps.

Thanks in advance