She does say to Charlie that she has to get a lot of things done in a short time, which if he didn’t know they were planning to flee, he might wonder at. It seems from the calls that they told Rash of their plans to flee, so I wonder whether he would have told Charlie.
But it seems to me like they didn’t want Charlie to know. Remember, when they called Charlie’s friend to ask about possible destinations, the specifically said “don’t tell Charlie.” (This person later reported this call which led to Donna’s arrest).
When they are talking about fleeing on these recordings, they don’t know they are being recorded and they think Charlie has hung up or been cut off. They are not talking to him. The initial reports that it was “jail calls” which led police to believe they were fleeing were misleading in this regard. They weren’t speaking to Charlie on a call.