My receptionist is telling me that there are a lot of occasions where she gets an incoming ring, the display screen says "Line X to Prime" and when she picks it up no one is there. What might be causing this? I thought maybe it was someone transferring a call back to the reception set and hanging up before the transfer is complete, but I'm thinking its not going to be that easy of an answer. Does anyone have an idea?
ETA, I can't see anything in the BCM settings.

Sounds like that line is hooked up to your system but is not programmed to AA or any set, therefore it times out to the prime set (recptn).
Find out what line X is then program it her set or AA.

Best to call a tech in to troubleshoot as well identify the phone number, or refer to your bill and call every line until that line rings.

You can use BCM Monitor/Lines as well.

Hmm, these are three new lines they added to the system a couple of months ago and no one's told me about this issue until now. Two of them (10 & 12) constantly do the "nobody's there after a ring" thing; I checked line 11 and the only difference I could see was that it had the reception DN on it. I have added the reception DN to the other two lines and we'll see if that does anything.

If a human is to answer the call then assigned it to the reception set under set programming, if AA is to answer then tell AA to answer it under vmail admin/lines.

Hi; here's a little more to add on here.
The reception set is to answer those lines so I have added them to her set. Nothing is to go to the AA when its incoming unless we are on the Night Ringing Service.
However; when anyone in the system picks up the phone, our system is set to pick the lowest line (highest line number) to make outgoing calls. However when we pick it up and its line 10, there is no dial tone. I checked the system and there is nothing different between line 10 and line 11 say. So when you pick up the phone and the system has chosen line 10 for you, you physically have to choose another line to make your call. Not sure why that is happening.
This morning, my receptionist is saying that when Lines 10, 11, 12 are incoming, and she picks them up; no one is there at all. Those lines are set to appear and ring at her terminal for incoming. I have tried adding them to one of the line pools and tried making them public lines, but the response is the same. No answer. The reception set is the control set for those three lines & they are programmed on her phone.
Does this additional info help at all?

Are you running PRI? or POTs?