Complete missing data in timeseries pandas
Don't forget you can chain operations (btw you missed the pandas import):  

sensed_time = pd.DataFrame(index=index, columns=['was_sensed'])
sensed_time = sensed_time.fillna(1)
sensed_time = sensed_time.sample(frac=0.6)

x = pd.DataFrame(index=index, columns=['was_sensed']).fillna(1).sample(frac=0.6)


You can verify if this works by doing a sensed_time.shape == x.shape

Regarding the construction of your code, it seems like it would be a better idea to have the data object as a class, and get properties from it.

For instance, something along the lines of:

    class SmartPhoneData(object):
        def __init__(self, name=None):
            self.name = name or "Nokia6_32GB"
            self.data = pd.DataFrame(index=index, columns=['was_sensed']).fillna(1).sample(frac=0.6)

        def calls_per_day(self):
            print(f"SmartPhone Data: {self.data}")

        @property
        def minutes(self):
            return self.data.reindex(pd.date_range(sensed_time.index.min().date(), sensed_time.index.max().date() + pd.DateOffset(1), freq='1Min'))

        @property
        def hours(self):
            return self.data.groupby([pd.Grouper(freq='1H')]).count()

        @property
        def days(self):
            return self.data.groupby([pd.Grouper(freq='1D')]).apply(label_sensed_days, hours_in_day=24, percentage_valid_hours=0.8, percentage_valid_minutes=0.8)


    your_phone = SmartPhoneData()
    my_phone = SmartPhoneData("iPhone X")

    your_phone.calls_per_day()
    my_phone.calls_per_day()


Obviously I just took a single line from your code where it was similarly named (to match the function).
So if you do this - make sure you know how the data gets changed when you run each function. For instance, each of the @properties just does a return and doesn't modify the DataFrame, however some of your options might actually change the data contents (even if there is no assignment (e.g.: x = a + b doesn't change a or b, but a (not real code) self.data(column_names = ["one", "two"]) will change it). I hope that made sense?

Good Luck!