Approximately what percentage of incoming calls fail? If it’s fairly high, e.g. 20% or more, you should be able to at least occasionally see a failure calling in from your own mobile and trace what happened.
If low, you could try running a continuous capture. When a failure is reported, look at what, if anything, was received about that time. See
If some failed calls occur while other calls are in progress and continue normally, unstable trunk connectivity is very unlikely.
If all failed calls occur while other calls are in progress, possibly the provider is not allowing as many concurrent calls as your contract specifies. You may be able to demonstrate this by having several associates call in from their mobiles at the same time.
Does Du provide a call log on their portal? If so, what, if anything, appears there for failed calls?