If internal calls made from extension to extension are suffering in terms of call quality - whereby a person cannot hear another person or there is interference on the line when an internal call transfer is done then the issue is caused by a local network issue as the calls are only going out via the internal local network. 

Speak to the person that manages the local network as the VoIP traffic in this instance is going over the 'internal' local network and not over the external ISP.