@Cyn..th...ia............call drop, engaged tone........  :-)))

 

As Mich has also pointed out, packet losses could be the main cause. To get away from QOS, jitter and the rest try to get a separate circuit for voip which will eliminate the issues, but first do check the for link drops and packet losses to establish cct parameters. Using QOS and the rest will affect quality at some point eg Monday morning when mail download  traffic is high. HTHs.