Unfortunately, there are any number of reasons this could be a problem. The only sure way to tell what's going on is to look at the SIP traffic directly.
If you are up for it, here are the steps you'll have to take:
1. Download and install Wireshark(it's free) from www.wireshark.org
2. Install wireshark on a PC and connect it to the same switch as the Tenor,
3. Configure the switch to use the PC port as a monitor port for the port the tenor is connected to; you need a managed switch for this.
4. Start a capture with Wireshark for the appropriate interface. You'll probably see a shocking amount of traffic on your network all of a sudden. Don't worry, we're going to filter out the stuff that doesn't matter and it will be much more manageable.
5. Click the Telephony tab and select "VOIP calls". Try to make a call or two and see if this provides any useful info.
6. If #5 didn't help, go back to Telephony and select the "SIP" filter. This will provide more basic info on what is going on with the Tenor by showing every SIP packet. If you don't know anything about SIP it can be pretty hard to read though - you may need to post results here and let us take a look.