david.moore.83,
I could be wrong, but it sounds as though that Hunt Group is an ACD Hunt Group and they have been logged out. Try this the next time it happens.
Dial 4+HG#, example: 42000 for HG 2000, and see if they retrieve the call. If they do then just log them back into the Hunt Group. You can also go to the Hunt Group and see if the ACD flag is checked.
System > Devices and Feature Codes > Hunt Groups > HG # > ACD Hunt Group: Yes
Thanks,
TE