This is a tricky, although very common problem when setting up VoIP circuits.
Quality issues are very often caused by latency on the IP network. If there are traffic “spikes” on the network this would explain why some calls are good and others are poor. This can be tested by running a ping command from the CLI on the Quintum to the unit you are connecting to.
Sometimes the wrong CODEC can cause problems but I don’t think this is the case here as this typically leads to no voice at all, although the call appears to be up.
First port of call – try the ping test periodically. The maximum you want to see is a 800ms round trip, any more and you will start experiencing quality issues.
If this is OK I’d be happy to look at the event log for you to see if any problems lie there.
Feel free to contact me at firstname.lastname@example.org