can’t communicate / Cisco and Quintum

I am setting up a Quintum Tenor D3200 with a Cisco Catalyst 2950
Gigabit Switch but I can’t make them to talk to each other. I tried using 3com hub and D-Link switch and my VOIP can see them. I also tried connecting it to a cisco 2924 switch and it works. I ruled out that there is no problem with my cables because it works with other switches. I dismissed the issue regarding the LAN switch not working because there are 3 VOIP’s connected there which are all working. I ruled out also that the problem might be the quintum itself because it works on other carriers. Is there a need to configure the Lan Switch or it’s just not compatible with my VOIP? Please help. Thanks!