Daisy apparently had some suspicion that Tom was being unfaithful. When Tom leaves to take the call, Daisy's first reaction is to try to cover the departure and carry on with conversation with Jordan and Nick, but she can't carry it off.
As if his absence quickened something within her, Daisy leaned forward again, her voice glowing and singing. "I love to see you at my table, Nick."...Then suddenly she threw her napkin on the table and excused herself and went into the house.
When Tom and Daisy rejoin Nick and Jordan, Daisy is the one who makes an excuse for Tom's phone call. Her conduct suggests that she's not pleased to be doing so, however. Her initial comment is delivered with "tense gayety" and is followed by meaningless comments about how "romantic" the outdoor view is, making a point to get Tom to agree about the romance of the setting.
When the phone rings again, Daisy is decisive in signaling to Tom that he was not to answer it again. She did not want him talking to whomever was calling.