Random Incoming call Issue, I'm stupmed

This is very random and has me stumped. I have a client setup and every so often someone will tell them they dialed in an no one picked up the call and the IVR did not pick up, they hung up and called a minute later and it was working their call was answered. Initially I thought well they may have dialed the wrong number. I was onsite yesterday making some other changes and I dialed in to test and it happened to me.
It has a Digium tdm400p card.

Anyone else have this problem or suggestions on how I would begin to troubleshoot.

Many Thanks