The first time Sarah talks to her mom on the phone, Sarah says it is four minutes 'til midnight. When she hangs up and looks at her phone to see the text alerts, her phone shows that it is nine minutes 'til midnight.
In the first loop, Sarah chastises her mother for calling at four minutes to midnight. Later, The Doctor says the time loop is only in the building. But Mammy's call arrives roughly the same time into each loop, meaning her supposedly-unaffected call is one minute later, just like the loops.