there can be multiple reasons why ur getting this error. Once of the most common reason for this to happen when you have still not exhausted the resources is you have a device pool which does not have access to the resource. For example, the phone is trying to request MoH with codec of g729, but the MoH server only supports G.711, this would cause an issue and the alert to be generated. Go to IP Voice Media Streaming Application Service Parameters configuration and select all codecs for MoH Audio Format and then restart IPVMS service and see if this resolves the issue
