The answer to this question would be yes, they are parameter.
Parameter is a data that can show the tendency of the sample. The example would be mean, mode, median. The parameter can be used to show how much similarity or differences between two sample.
Formula to find the slope with two points: m=y2-y1/x2-x1, from the two points you were given (-2,-3) and (1,1). So m=1-(-3)/1-(-2)=4/3
Answer: the independent variable would be m (minutes spent talking on the phone)
Explanation: the cell phone bill will be affected by how many minutes is spent talking on the phone. However the nothing will affect how many minutes is spent on the phone.