I have a issue with low volume external calls coming in. The calls are delivered to the company via an ip ip gateway from the service provider(H323 to H323). The ip phones are then located across WAN links at remote sites.
Only 2 out of approximately 10 sites are affected (but its difficult to assess as externals callers have to alert the called user). The handsets as these two sites are different but are 7911 at one and 7912 at the other. Additionally LAN to LAN calls are not affected within the network.
The issue seems to be more of an issue related to the PSTN, which is obviously located within the provider network. The service provider has claimed to have checked this and found nothing wrong.
Does anyone have any ideas how to prove that the issue is located in the SP network or even better had and resolved the issue otherwise?