You probably have at least two public IPs, one for the Endpoint and one or more for LAN users.
They could prioritize all traffic to/from the endpoint IP over the rest of the traffic. (depending on how it is configured, you can disrupt LAN users traffic).
If they can put more details, you can ask to prioritize RTP/RTCP packets to/from the Endpoint and if posible, on the TCP/UDP ports used by the endpoint. (then you can include H323/SIP signaling ports).
Hope it helps.
Regards