Missing some Inbound calls

[ScotttB]

This is not specifically IP Office, but I have noticed that some customers are not receiving all their calls. Here is an example, I can call a DID that is routed via a PRI on a 500v2 control unit, and some of the calls complete and some don't. From my Verizon Cell phone it works, but from a few SIP providers I have access to test with, it's either intermittent or not at all. The customers carrier says that the number/s I was calling from never hit their switch, they never saw them. I have made calls, and I'm receiving, "This call cannot be completed as dialed." Even though I know the call was dialed correctly and I just hit redial a few times and the call went. A couple of times I would call a number, expecting it to go to VM Pro but I never see the call hit the IP Office, and I get a message telling me I reached voicemail of the EXACT number I dialed. It was like a default cell phone type message. Then I call from another carrier and the call completes as normal.
I'm really just looking for feedback to see if anyone else has noticed weird or odd things. Or am I in the Twilight Zone?

Thanks