Call is an old term. You used to find many references to "calls or orders" in the pre-FAR procurement regulations. As best I can determine the word call was not officially defined. In my experience it was used in connection with Blanket Purchase Agreements. Agencies would place a call against a BPA. The term originally may have literally referred to a telephone call, which was how most BPA calls were made. It may be that calls were placed against agreements, like BOAs and BPAs, and orders were placed against contracts. You placed a call when the contractor was not contractually bound to perform and an order when it was. But that's speculation.