I can' really think of a reason that would come up for you to call an agent/manager. Usually they are involved in $$ type of discussions, and as SM I want no part of that.
The agent/manager may negotiate on behalf of the actor for benefits (better housing, a private car, special accomodations for their dressing room, etc) but any information you need to know (eg, dressing room requests) should come to you from the company management/casting departments/producer, depending on the structure of your theatre/company. Company Management, casting, producers, press department might need to call the agents so if you want to add their numbers to the contact sheet for their benefit you can... at the regional level I usually leave that off.