Help With Remote User

We have a Trixbox appliance at our office and we have a remote user who we have been having inbound voice issues with. Internally our voice and data networks are separate. The Trixbox appliance is outside of our firewall. It has PRI connection to a Cisco IAD which is in turn connected to 2 bonded T1's. The Cisco IAD also has an ethernet connection to our data network so both regular data and voip go through this box. We use Cisco 7960 hardphones. We have no issue with voice quality on calls made and received by phones physically at the same location as our Trixbox. We have a remote employee who has a cable modem for access into our network. He has a Cisco hardphone that connects to the external IP interface of our Trixbox. When he calls into our office his voice quality often becomes very choppy. He never has trouble hearing us and when he calls other external phones he has not had any reports of bad voice quality. We have tried a couple of different ISPs for him, different methods of connecting to his modem and enabling QoS features on his router using Tomato firmware. We have tried a traffic shaper on our network to limit our overall data bandwidth and nothing has made a difference. Our business T1 supplier has told us that QoS is not being applied to incoming data and according to them we would need MPLS service to have this done (not cost effective). It is important to our business that our remote employee is on the same phone network (he is on ring groups, etc.). We are running Asterisk 1.2.24 and Trixbox 2.2.10. Does anybody have any suggestions for further debug of this problem or a suggested configuration that might make this work? It seems to me that this should be a workable configuration, but we have multiple service providers all pointing their fingers at each other. Thanks.