I have a 670 rel 3.10, with ACD. When a agent transfers a call and the call recalls because of a no answer or whatever, the agent is "logged out ??" of ACD. The log in light remains on, and the unavailable light is off, however they can no longer receive any calls until they log out and back in. I can not reproduce this trouble, however they have been complaining about it for quit some time.

Do the agents have any trunk appearances on their phone?

No, only DN's and DSS keys

Try upgrading to newest ACD software. Then I would call Oaisys if problem continues.