Assuming the rate of phone calls is constant at 425 calls per month, you would have a proportional relationship between time and number of phone calls. In other words, the total phone calls would be dependent upon the number of months. i.e. 1 month = 425 calls, 2 months = 850 calls, and so on....
Therefore, for this relationship between time and total calls the following would be true.
The time( in months) would be considered the independent variable.
The number of calls for a specified number of months would be considered the dependent variable.