What bothered the original poster is whether calls are received an order (with no acknowledgement order is something calls rest IN).
Sounds stupid, right? You are being reassured - the calls in queue will be answered in an order - however, is their present order not indeed "the order IN which they were received"? We think of them being IN an order. Think about whether you would puzzle with other tourists over a sign reading, "We ask that you return to the hotel in the bus you arrived. (end of sentence)"? We are accustomed to thinking about order in terms of a set, "into" which things come and "out of" which they go, and our ears expect the relationship to be stated as of one thing "IN WHICH" another thing resides. The examples here seem missing something.
No opinion though, personally, as to whether one is entitled to hear about calls being received "IN order".